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Email troubleshooting on the Mac covering sending failures, account setup, syncing problems, attachments and Outlook-specific errors.
Updated May 30, 2026
Email problems divide cleanly into two categories: messages that will not send, and messages that will not arrive. Diagnosing either means understanding that mail passes through your app, your account provider and the recipient's provider, and any of the three can stop it.
This category covers account setup and authentication, sending failures, syncing behaviour across devices, attachment limits and the mailbox problems that appear in long-running accounts.
Where a step involves removing an account from a device, we flag whether locally stored messages are at risk first.
Sending failures typically involve authentication, outgoing server settings or the mail service itself.
Check the account type and sync settings before assuming messages were deleted.
Modern mail services require app-specific handling for sign-in and stored credentials go stale.
Size limits are set by mail providers on both ends, not by your Mac.
Error 17099 in Outlook for Mac indicates a problem sending email. Learn what causes this error, how to check account settings and network access, and when to involve your IT team.
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