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How to fix SMTP Error 510
Repeated 510 errors damage your sender reputation, increase hard bounce rates, and raise red flags with ISPs and ESPs, which can lead to spam filtering, throttling, or blocking. In this guide, we explain the nature of the error, its reasons, and ways to fix it.
This error is triggered when the Simple Mail Transfer Protocol (SMTP) server attempts to verify the recipient’s email address and finds that it’s malformed, non-existent, or no longer active.
What causes email addresses to be invalid?
1. Human error (input & data collection issues)
- Typos in domains or usernames (e.g. gmial.com instead of gmail.com)
- Manual entry errors such as extra characters, missing dots, or wrong extensions.
- Misheard addresses from verbal exchanges
It's important to validate the emails in your lists before starting outreach to avoid this error. You can use the Warmy Email Validation Tool to run checks beforehand and ensure that emails in your list are valid.
2. Organizational change
Email addresses can be invalid when the owner has changes due to business reasons like:
- Job turnover (employees leaving companies)
- Company rebrands (changing company name often leads to change in domains and email addresses used by employees)
- Mergers and acquisitions
- Domain migrations and domain shutdowns
3. Data decay or list aging
Around 25-30% of email addresses on a list become invalid, inactive, or unusable within the span of a year. These are due to:
- Outdated contact lists
- Abandoned inboxes
- Deactivated accounts
- Recycled email addresses by providers
4. Infrastructure-level traps
These are addresses specifically created to detect bad sending behavior:
- Spam trap addresses
- Honeypot accounts created by ISPs and anti-spam organizations
- Scraped or purchased list contamination
5. Segment your lists
Base them on engagement level and validity. Keep your most active and verified contacts separate from questionable or older addresses. This way, if you want to attempt reaching less-engaged contacts, you can do so with a separate sending infrastructure that won’t affect your primary sender reputation.