DIAGNOSTIC PLAYBOOK
Email rejected for message size: inspect encoded size
Message-size limits apply to the transmitted message, including encoding and MIME overhead. An attachment's file size alone can underestimate the size the receiving server evaluates.
HealthCheck Email editorial team · · Examples are illustrative
How to investigate
- Read the exact size-related response and identify the rejecting server.
- Measure the complete encoded message rather than only its attachment files.
- Reduce or replace oversized content with an appropriate supported delivery method and test a fresh message.
What this looks like
ILLUSTRATIVE EXAMPLE
An attachment expands during transfer encoding, taking the full message beyond a receiver's limit even though the original file appeared small enough.
A mistake to avoid
Do not repeatedly retry an unchanged oversized message after a permanent rejection. Correct the content first.
Keep the result in context
A successful handoff, a delivered event and an inbox placement are different observations. Record the receiving provider, send time, SMTP response and affected stream before comparing results. DNS fixes address authentication or routing; they cannot establish consent or guarantee placement.
Take the next step
Use the related check to gather evidence, then compare it with the affected message or service. Keep the result and time with your notes so a later change can be distinguished from the original problem.
Sources and further reading
The protocol references below explain the underlying behavior. Your sending or DNS provider supplies the account-specific settings for its service.