Why did my campaigns pause?

A campaign that pauses itself always says why. Open it: the reason is shown at the top of the page, and a red banner appears on every page while something is paused. These are the possible reasons.

Reasons and what to do

MessageWhat it meansWhat to do
Paused to check your list: N of the last M emails bouncedThe bounce checkpoint. More than 2% of recent emails bounced.Look at the addresses that bounced, then answer the question on the campaign page. See Bounce protection.
Paused: this mailbox's emails bounced N% over the last 48 hoursThe mailbox ceiling (4%). Every campaign on the mailbox stopped together.Clean your lists. It can resume once the rate falls, which happens by itself within 48 hours of the last bounce.
Sending problem — N of the last M sends failed, and paused automatically because sends were failingThe mail server refused the emails outright (not bounces that came back later). The card shows the server's own error text and what it usually means.Fix what the error says — see below — then resume.
The sending account is paused after an errorThe whole mailbox was refused (for example Amazon SES paused your account) or its login broke.Fix the provider problem or reconnect the mailbox, then resume it on the Accounts page.
Paused manuallySomeone pressed Pause or Stop.Resume it when ready.

Common server errors

Sending paused for this accountAmazon SES has paused sending for your AWS account, usually because too many emails bounced. Nothing will send until they re-enable it. See If SES pauses your account.
Email address is not verifiedThe From address or domain isn't verified with the provider, or the account is still in the sandbox.
Authentication failed / 535The mailbox password was rejected. Use Edit connection on the Accounts page to enter it again.
550 5.1.1 user unknownA dead address. A few are normal; many mean the list needs cleaning.
Quota / rate limit / too manyThe provider's sending limit was reached. Lower the mailbox's daily limit or batch size.

Every failed send and its reason is listed under the campaign's Failed events (click the number in the dashboard, or use the link in the “Sending problem” card).