Why Are My Emails Bouncing? A Practical Troubleshooting Guide

A bounce message can look intimidating - a wall of server codes and unfamiliar terminology - but the underlying cause is almost always one of a small number of possibilities. Working through them in order, from most to least likely, is faster than reading the full bounce message line by line.

Hard bounce vs. soft bounce

A hard bounce means the address is permanently undeliverable - it does not exist, or the domain cannot receive mail at all - and should be removed from a list immediately; repeatedly sending to it damages sender reputation. A soft bounce is temporary - a full mailbox, a server that is briefly down, a message that was too large - and is generally safe to retry later.

A practical order to check things in

1. Confirm the address itself is correctly formatted and was not mistyped - the simplest cause is also the most common.
2. Check whether the recipient's domain has valid MX records at all - no MX record means every address at that domain will bounce, regardless of the specific mailbox.
3. If the domain is yours and the recipient is elsewhere, check your own domain and sending IP against major blacklists - a listing causes exactly this symptom.
4. Confirm your own SPF, DKIM and DMARC records are correctly configured - a receiving server rejecting unauthenticated mail produces a bounce that looks identical to other causes from the sender's side.

Preventing the next one

Most future bounces are preventable by verifying addresses before sending rather than diagnosing them after the fact - catching an invalid address at the point of entry costs nothing, while a bounce after sending has already spent some of your domain's reputation.