Issue with sent mailbox

Hmm Server Unavailable seems to indicate that no action would succeed, but other commands will have succeeded if you get to the APPEND command.

The command itself indicates that MailMate is trying to upload an email which is in your “Sent Items” mailbox of the account. You might want to locate that and delete it to see if it some triggers a server issue. Note that O365 has the non-standard behavior of saving a copy of sent messages itself (when using SMTP) and therefore you should have another (slightly different) copy of the message.

You can identify non-uploaded messages in the Sent Items folder by enabling the UID column in the message list. Any message without a UID only exists locally and has not yet been uploaded.

Let me know if this does not help.