Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors. You can retry or save the message locally to Local Folders/Sent-
I have suddenly started receiving this message when trying to send emails. They are just plain emails with no attachments. Thunderbird is not "getting" any emails either. I did not change 'ANYTHING' in Thunderbird; it just started happening. I've tried the possible solutions I've found on line but haven't solved the problem. I'm using Windows 11 and Network Solutions email.
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If Thunderbird cannot connect to the (IMAP) incoming mail server, that would also be the reason sent messages cannot be saved, because even though the messages are sent using the SMTP server, which you appear to be able to do, all messages, be they incoming or outgoing, are stored on the IMAP server.
Don't you get any error messages when checking for new mail? Post a screenshot of your incoming mail Server Settings, in case there is something amiss there.
Read the following article in the meantime to see possible causes of the problem:
https://support.mozilla.org/kb/cannot-receive-messages
In particular, if you're using a VPN, disable it and see whether that makes the problem go away. If it does, check whether your VPN has an option called split tunneling which would allow you to let Thunderbird bypass the VPN.