Have you set up the account in Thunderbird on the PC as POP or as IMAP? If POP, it would be working as expected…

Hi David,

Thank you for the suggestion. The Server Setting is indeed set to IMAP Mail Server.

I should add that I have been running this set up for many years and this odd behavior only started recently, in the past couple of weeks I would say. I'm running Thunderbird 52.9.1 on my PC, which I see is from 2018. Perhaps I should update to whatever the latest release is.

Dan

A further check would be to check online to see where the message is. That should confirm what is happening. If you do upgrade, be sensitive that Thunderbird does maintenance changes on release updates. That is, leaping to the current version will cause problems. Upgrading, step by step, to the initial version of each release is the safest approach. All listings are here: https://www.thunderbird.net/en-US/thunderbird/releases/ It's possible to shortcut this, but this is safest way.

Thank you david,

I logged onto my email provider's webmail service and compared the messages in my inbox there to the messages in the inbox on my iphone and Thunderbird on my PC. The webmail inbox is the same as the iphone inbox but different from my Thunderbird inbox - so I conclude that when I move messages in Thunderbird from the inbox to folders in the Local Folders, something is going awry. Thunderbird thinks the messages is moved, but somehow the signal back to the email provider to tell them that is getting lost.

In any case I will go ahead and upgrade, I am on the release update channel for which 60.9.1 is apparently the latest. Since release 60 is the next release after 52, I will upgrade to the initial version of 60, which is 60.2.1, then upgrade to 60.9.1. Thank you for pointing that out to me.

Dan