Seeing "authentication failed", a 535 error, or an IMAP connection failure when connecting a mailbox to your sequencer? Try these in order:
Reconnect the mailbox in your sequencer using SMTP port 587 with TLS enabled.
Re-export a fresh CSV from the Email accounts page and re-add the mailbox with those exact details (IMAP 993 with SSL on, SMTP 587).
If it still fails after a fresh reconnect, it may be a temporary server-side issue on our end — contact support and we'll look into it.