Error when trying to forward mail

eM client seems to work fine 99% of the time, but occasionally when trying to forward a mail I get the little red triangle and “An error occurred (IMAP) Message temporarily unavailable”.
The message goes into the sent box but the red triangle never goes away.(I think the mail is actually sent.)
Log of the most recent
3:13:46 PM Videotron (deforestradio) IMAP Synchronizing folder ‘/Sent’
3:13:53 PM Videotron (deforestradio) IMAP Synchronizing folder ‘/Sent’
3:14:20 PM Videotron (deforestradio) IMAP Uploading item(s) to folder ‘/Outbox’
3:14:20 PM Videotron (deforestradio) SMTP Connecting: To Videotron (deforestradio)
3:14:23 PM Videotron (deforestradio) SMTP Connected: To Videotron (deforestradio)
3:14:23 PM Videotron (deforestradio) SMTP Sending messages
3:14:23 PM Videotron (deforestradio) SMTP Disconnected: From Videotron (deforestradio)
3:14:23 PM Videotron (deforestradio) SMTP Disconnected: Done
3:14:23 PM Videotron (dnjcheney) IMAP Synchronizing folder ‘/Inbox’
3:14:38 PM Videotron (deforestradio) [IMAP] MailClient.Storage.Application.ItemOperationException: Message temporarily unavailable
GUIStatus_Exception_Reported = True

Any help would be appreciated - when I see that red triangle I am never sure if the mail was sent correctly.