What is a "catch-all" email address?
Healthcare email addresses have an unusually high incidence of catch-all or accept-all verification results in part because hospitals, health systems, medical groups, and academic medical centers often use email infrastructure that deliberately prevents outside systems from determining whether an individual mailbox exists. Catch-all servers may be configured to conceal mailbox validity during SMTP verification to protect against email address harvesting. Naturally, this is a major reason for the typically high bounce rates on cold email campaigns targeting healthcare providers.
Why are there so many healthcare email servers configured as catch-all?
Cold email campaigns targeting healthcare providers have typical bounce rates much higher than other industry targets. Marketers in the healthcare space report between 15% and 35% bounces as a normal bounce rate. Curious, right?
Because healthcare organizations are frequent targets of phishing, ransomware, and credential attacks, they take extraordinary steps to manage incoming messages.
In other industries, it is common for an SMTP server to be configured to respond to an unknown address with "550 5.1.1 User unknown," a response that allows an attacker to test thousands of potential addresses and identify real employees. There are both security reasons, and reasons related to preventing business intelligence from being gathered on highly competitive hospital systems or health systems – the U.S. healthcare industry, a vigorously profit-driven enterprise, is one of the most highly competitive such industries in the world. So, an organization may initially accept recipient addresses or return ambiguous responses, preventing outside parties from identifying its employees, the number and taxonomy of employees, and the relative strengths of its employees across the Provider Taxonomy index. But there are subtleties here, and a good email vendor can judge from the specifics of a response, as well as historical evidence, whether any given catch-all address is good or not.
Microsoft 365 and enterprise email security
Large healthcare organizations frequently use Microsoft 365, Exchange Online Protection, Proofpoint, Mimecast, and other enterprise email security systems. These systems may:
- Accept or defer SMTP recipient validation,
- Hide whether a particular physician's mailbox exists,
- Route messages through security gateways before evaluating the recipient,
- Return temporary or ambiguous responses to verification probes.
As a result, email validation services may classify the entire domain as catch-all, even when a majority of the addresses will reach inboxes.
Large scale hospital email systems
Centralized hospital email systems face additional challenges that make catch-all email attractive. A large health system may operate one email domain serving tens of thousands of employees, including physicians, nurses, administrators, and contractors. That makes it difficult to handle the volume of employee email removals when people leave the organization. Automation here is a challenge. Centralized routing, directory synchronization, aliases, and forwarding rules make it difficult for internal personnel to keep up, and for external verification services to determine which individual addresses are deliverable, unless evidentiary techniques are engaged.
Catch-all and Security
Cybersecurity safety presents another incentive for healthcare organizations to use a catch-all approach. They have strong incentives to avoid revealing employee account information. HIPAA does not specifically require catch-all email configurations, but healthcare cybersecurity programs often discourage exposing valid usernames and email accounts to unauthenticated external systems.
What does "catch-all" actually mean for physician email quality?
| Verification result | Interpretation | Deliverability confidence |
| Valid | Mailbox appears to exist | High |
| Catch-all | Mailbox existence may not be confirmed | Unknown, unless additional evidence of delivery is present |
| Invalid | Address rejected or otherwise determined nonexistent | Very low, do not use |
| Unknown | Verification could not be completed | Unknown, shouldn't be used unless additional evidence or delivery is present |
Evidentiary Techniques to Identify "Good" Catch-all Email Addresses
A catch-all classification is not evidence that an email address is bad. In fact, an authentic clinician address at a major hospital may be classified catch-all while an incorrectly inferred address at the same institution receives exactly the same classification. In addition to basic SMTP verification, a stronger validation process combines SMTP testing with independent evidence, such as:
- Historical successful delivery and hard-bounce records. CarePrecise's relationships with large email senders offers the source for this evidence.
- Confirmation from institutional physician directories. A large percentage of CarePrecise email addresses are captured in this way, and the directories are used to cross check emails.
- Consistency between the physician's name, organization, and email domain. The vast majority of CarePrecise email addresses meet this criteria. Because CarePrecise maintains a comprehensive database of all 9.4 millionU.S. healthcare providers – from health systems to physicians, and nurses, to ambulance drivers.
- Evidence that the physician remains affiliated with the organization. CarePrecise performs this check with every email re-verification, based on monthly clinician location updates.
- Recent email engagement. CarePrecise clients share bounce reports, which gives evidence of opens and related quality and ROI data.
A particularly useful distinction is between an address that is merely syntactically plausible at a catch-all domain and an address that has previously been demonstrated to receive mail. The prevalence of catch-all results can make physician and other clinician email databases look substantially worse than they actually are when measured using conventional email verification tools.
For example, a database with 25% catch-all classifications does not necessarily have a 25% uncertainty rate in real-world deliverability if independent delivery evidence exists for those addresses. The key metric is actual hard-bounce rate, not the percentage of addresses classified catch-all. If you're evaluating healthcare provider email database quality, a CarePrecise solutions analyst can explain how our email addresses are graded for superior effectiveness, catch-all or not.
What do I do if my ESP refuses to send because my bounce rate is too high?
This happens frequently with healthcare email campaigns, especially when you haven't warmed with non-catch-all addresses first. Many Email Service Providers (ESPs), like MailChimp, ConstantContact, and others, don't understand the differences between healthcare email and other industries. In particular, the hospital and clinician segments handle incoming email very differently from industries like insurance, real estate, technology, and most others.
If your email service provider tells you you have too many bounces, you're not alone. Even with opt-in email lists developed buy your own company, a larger-than-average number of the addresses will be "catch-all," which triggers an ESP to believe that you're sending low quality bulk email. In healthcare, there is no way to avoid sending to catch-all email addresses (see ""). You will do well to use a service that is more open minded.
Standard marketing ESPs (like Mailchimp, Klaviyo, or ActiveCampaign) rely heavily on pre-send list hygiene algorithms or third-party list-cleaning APIs. When their automated filters see a list dominated by "risky" or "unverifiable" catch-all domains, they flag or block the upload to protect their shared IP pools. To deliver to these domains without getting blocked by your email provider, you need platforms built for B2B sending or infrastructure that allows you to control your sending policies.
If your list consists of healthcare prospecting data rather than double-opt-in subscribers, standard ESPs will continue to ban you regardless of catch-all status because third-party lists violate their Terms of Service. Instead, use cold outreach platforms designed to send via distributed inboxes.
Look at Smartlead.ai or Instantly.ai. These platforms connect directly to Google Workspace or Microsoft 365 accounts rather than sending via mass SMTP relays. They do not block lists with catch-all addresses, and sending via native enterprise mailboxes typically yields higher inbox placement against hospital security gateways. Even if an ESP allows you to send, healthcare catch-all servers will eventually reject invalid mailboxes asynchronously, which triggers hard bounces after the fact. If your bounce rate climbs above 3–5%, Google, Microsoft, and healthcare spam filters will block your sending domain entirely. Instead of running standard SMTP verification (which fails on catch-all domains), use tools that specialize in resolving accept-all mailboxes. Some of your options for sending cold email:
- For cold prospecting: Use Smartlead or Instantly.ai paired with BounceBan or Scrubby to filter out dead healthcare addresses before sending through dedicated secondary domains. You email vendor may be willing to replace all or a portion of these, based on a genuine report from the service that explicitly states that the addresses are not valid. CarePrecise is one of the few who will do this for you, realizing that even its own verification system can be wrong sometimes. Note that email vendors may not accept the very basic reports produced services like ZeroBounce, and usually require established reason codes, not just "this email address is undeliverable," but clear, cold codes that describe the reason for the bounce in terms more detailed than "soft bounce" or "hard bounce." Vendors can't be expected to take responsibility for email that is undelivered because of the sending domain or message content, or because the campaign skipped the critical "warming" protocol. Preserve separate bounce reports from each of the small sends as proof of warming.
- For opted-in or transactional delivery: Use Mailgun or Amazon SES with a dedicated IP address, bypassing the conservative list-blocking algorithms of retail marketing platforms.
- Scrubby.io: Specifically designed for lists that other validators mark as "risky" or "catch-all." It validates whether an address actually exists behind an accept-all firewall without burning your main domain.
- BounceBan: Capable of verifying a large percentage of catch-all and accept-all domains using proprietary AI and SMTP behavioral modeling.
- Clearout: Offers specialized B2B confidence scoring for accept-all addresses to identify which ones are likely valid before you load them into your delivery tool.
- Dedicated Cold Email and Private SMTP Providers: These providers cater to outbound cold outreach where catch-all addresses and unverified data are common. They do not shut accounts down as aggressively as marketing ESPs, provided you are on isolated infrastructure:
– Maildoso / Infraforge / Superwave: Built specifically for outbound infrastructure. Rather than sharing reputation with thousands of users, they provide dedicated or rotated IPs and private mailboxes configured for outbound volume.
– Dedicated IP SMTP; e.g., dedicated nodes on Postmark, Mailersend, or custom VPS (virtual private server) relays: If you lease an isolated, dedicated IP address, your high bounce rate will damage only your own IP and domain reputation (which is why you use this isolated IP instead of your corporate domain), rather than affecting other customers. While providers will still step in if spam traps or abuse complaints spike radically, you bypass shared-pool bounce enforcement. A legitimate outreach campaign to healthcare providers will not be likely to trigger alarms unless the message in the email is inciting or highly evocative.
CarePrecise does not recommend specific vendors or services. Examples above are given for illustration purposes only; other vendors may offer similar services.
Finally, It's About Scaling
Ultimately, the bare fact of so may catch-all addresses in the U.S. healthcare provider space presents a challenge, and results in higher bounce rates than many marketers are accustomed to, regardless of masterful curation and culling. Accepting this fact, and scaling campaigns to account for it, is key to successfully reaching clinicians and organizations with a cold email campaign. Larger sends, expecting 15% to 30% bounce rates, are necessary; the best email vendors offer volume discounts on email purchases, as well as liberal use policies that permit re-sending multiple times. Establishing a relationship with an ESP that offers liberal send policies, and/or creating your own IP, is frequently a necessity.
Author: Michael Christopher
Michael is a Solutions Analyst, Chief Product Engineer and Partner at CarePrecise, bringing thirty years of experience in managing enterprise scale email campaigns in the healthcare and nonprofit business space. Along with early business partner Martin Jensen, Michael served as a healthcare IT consultant, developed and edited an award-winning HIT blog and email newsletter, and authored papers presentations for the Workgroup for Electronic Data Interchange (WEDI) and Healthcare Information and Management Systems Society, Inc. (HIMSS). He has served on committees and boards related to nonprofit and healthcare business development. As for hands-on technology development has built complex email capture and verification systems, verification automation, healthcare provider contact data curation technology, and provider data aggregation, normalization and data product development. Michael has been a Partner in CarePrecise LLC since its initial planning and inception in 2007, and manages contact data acquisition and curation. He is grateful for the contributions of colleagues in healthcare business development.
Check our email best practices in the CarePrecise Email Manual: Get the Best Return on Your Email Investment.
