sending of the message failed. there was an error attaching file. please check that you have access to the file

You're seeing a bug that has been mentioned here recently. I'm looking at Bugzilla to see if it's been officially reported. I don't see it yet, but I'm still looking. https://bugzilla.mozilla.org/describecomponents.cgi?product=Thunderbird Version 152.01 is out now. I don't know if that bug is fixed there but I'll test it later. (later edit...) The bug appears to NOT be fixed in 152.01 . What I see happening is that when you create a Forward message that has attachments, copies of the original attachment are placed in Windows' %TEMP% folder. When you eventually send the message, those temporary copies of the attachments are attached to the message and they're on their way. And that's what happens if you create the message and send it within a minute or two. However, after some several minutes, the attachment copies disappear from the TEMP folder; if you try to send your message after that time, the attachments can't be found and you get that error message. This is obviously wrong behavior.