What the receiver is telling you

The message is bigger than the receiver, or the recipient's own mailbox policy, allows. Gmail's limit is 25 MB; Exchange defaults to 35 MB but tenants and individual mailboxes can be set far lower — 10 MB is common in regulated industries. In cold outreach this is nearly always an attachment: a PDF deck, a case study, an embedded video. Anything over a couple of megabytes in a first-touch message is also a spam signal before it is a size problem.

What a hard bounce means for your reputation

Receivers count hard bounces. Gmail and Microsoft both treat a sender whose mail is addressed to people who do not exist as a sender who did not get those addresses from the people themselves, which is the definition of unsolicited. The working threshold is two percent of a send: under that is noise, above it starts to cost domain reputation, and above five percent a fresh domain can be throttled after a single campaign. The fix is upstream — verify before sending — not downstream.

What to do

1. Drop the attachment and link to it instead — a hosted PDF on your own domain.
2. If you must attach, keep the whole message under 5 MB; that clears every common limit.
3. Check the HTML template for embedded base64 images, which bloat size invisibly.

don't Do not retry the same message. It will fail the same way, and Exchange logs repeated oversize attempts.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions