Are you using IMAP with Gmail or Outlook? If so, make sure to uncheck "Place a copy in" in the Copies & Folders section of the Account Settings.

In case you do need that setting in order to keep copies of any sent messages (which should be managed by the mail server, hence causing issues), leave it checked but select the folder using the "Other" setting below that instead.

Does this help?

Thank you Tonnes. That is progress but not yet solved.

I am using IMAP but not with Gmail or Outlook, with mail.gridhost.co.uk.

I do need to keep copies of sent messages so I have used the 'other' option you suggested but even though I specify it should save a copy in the sent folder it places it in the inbox. As a temporary workaround I can then move it to the sent folder but this seems a very odd problem. Surely everyone needs to keep copies of sent messages?

The copy option can be valid for other email providers offering IMAP so I mentioned it just in case. It also depends on whether or not one is using that provider’s SMTP server to send mail - since outgoing messages will pass by anyway, they can copy these messages to a Sent folder on the server automatically, which will then be synchronized back to Thunderbird’s Sent folder. This would become clear when enabling it - you might see double sent items - and considering the additional traffic it would cause when enabled, that could cause the delay. I’m not entirely sure but theoretically a server could delete or block such "additional" copy actions during that process, also causing delay.

- Does the issue also occur if you disable that option? (Just for testing) - Have you disabled virus scanning in any security software, and what software are you using? - I found some info claiming that mail3.gridhost.co.uk would be the proper server to use. Is that correct and if not, have you tried to change it? I know both point to the same IP, but if "mail3" is recommended, external aliasing issues may cause problems for users.