And how are you getting the messages to your alternative account. My guess is that process is what is making them disappear from your server synchronized IMAP account

Matt, Thanks for responding.I have Ubuntu and Windows set up on a divided drive and have had this configuration for years.In the past,when I get a message on Thunderbird it also goes to Windows Outlook.Both systems now fail to retain these receipts but I'm still able to send from both.When I noticed this I created a separate Gmail account which stands alone and which I have not as yet added to Thunderbird as my preferred.If I'm running a test where I send myself a message or I happen to be lucky and am logged in to Thunderbird when I receive a message I have about a minute to forward it to my Gmail to read later.Most times I'm not that lucky so the email simply disappears.I'm suspecting that this is a Cogeco issue since its the only thing constant between Thunderbird and Outlook and I have learned that the problem has forced one organization locally to advise it's clients to switch to something other than Cogeco.This may be my next step but it does not explain why it occurred right after the Ubuntu upgrade since all protocols seem to be in place.I'm hoping ThunderBird can shed some light. Thanks again.John

Agree with Matt Either you are sending all Cogeco address emails to gmail account, so they are no longer on Cogeco address server or you have accessed the server using another device eg: phone and downloaded using a pop account which has auto deleted them off the server when they got downloaded. Pop account would need to have the setting to 'Leave message on server' so that the imap account for Cogeco address in Thunderbird can still see them.

Do you also have a phone connected to that account? This sounds a lot like a pop phone collecting mail and deleting it in the server so Thunderbird and outlook IMAP account get the mail and they synchronize it into oblivion because the other device has deleted it from the server.

I have also seen very similar things when folks set up say gmail to collect mail, it uses pop and deletes from the source, as do most web mails that collect mail from other providers.

I would suggest you check in Thunderbird that the view menu > messages menu entry is set to all. Being set to something like unread can make things appear to disappear if the message status is changed for some reason.