What the receiver is telling you

One message was addressed to more recipients than the receiver allows on a single transaction — Gmail caps at 100 per message for external senders, Exchange defaults to 500 but tenants set lower. In cold email this should never happen: a sequence tool sends one message per recipient. When it does appear it means someone pasted a list into the To or Bcc of a normal mail client, which is also the single fastest way to get a Workspace account suspended.

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. Send one message per recipient. Every sending tool does this by default.
2. If it was a genuine group email, split into batches under 50.
3. Check that nobody on the team is running outreach from a shared inbox with Bcc — it will trip Google's bulk-sending detection.

don't Do not raise the batch size and retry. The limit is the receiver's and it will not move.

Decode the whole bounce

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

Common questions