This happens intermittently on Thunderbird 68 in both 32 bit & 64 bit PCs. Happens for both email with and without attachment. The error message is: "Your message was sent but a copy was not placed in your sent folder (Sent Mail) due to network or file access errors. You can retry or save the message locally to Local Folders/Sent Mail-xxxxxxx@aussiebroadband.com.au"

This happened when I upgraded to TB 68 so I have gone back to TB 60 and the problem has not reoccurred so far. I had a similar problem before that was caused by the antivirus software email scanning. Since that time, I have the antivirus email scanning switched off for both sending and receiving. So this problem is not caused by the antivirus software.