have you tried disabling your anti virus mail scanner? they are the usual cause of this sort of issue.

Sometime just rebooting the computer will do, but removing mail scanning is the next step. What anti virus do you use?

Thanks for the response Matt. Yes did all of that, a number of times. Where is Tbird finding the information it puts onto the log files? Same rubbish each time and not from my ISP. Cleared Tbird cache, closed Tbird, cleared W10 cache, Reboot This sequence repeated twice with no effect. Also re-installed on top of existing application.

For Tbird to be writing that data in to the log file it must be storing it somewhere. If I knew where I could delete the file or over write it with a cleaned copy.

I re disabled my Avast this morning. This time it has downloaded the emails plus the phantom email. Re enabled Avast operations , squirted test mail, which was received correctly. I will not be back here for 48 hours or so. That will be the real test. If all OK I will close this question. Thank you for reading.

OK 36hrs later. All good. Been thinking what has changed since I last disabled AVAST. The main change may be the update to 6.3.2. Of course other updates etc could have change the environment.

After over half a century in the business I know that there are very few definitives in complex environments.

Thank you to Matt for responding and all who read it.