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Today, one of our clients (who is on a DOT account), received a message which Outlook flagged as spam/potentially malicious. He didn't open any attachments.
Ever since then, his Outlook has a constant error at bottom right:
! Task 'firstname.lastname@example.org - Sending' reported error (0x800CCC61): "Your outgoing (SMTP) email server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: 558 5A204A960D1AA6 Message content rejected due to suspected spam. OB703
He is able to send and receive all other email fine, except when we press send/receive, it tries to send 1 email. There are no emails in the outbox.