No easy answer. Needs some detective work. The easy approach is to see if both phone and PC are using IMAP. If either is not, that's the problem. If you still don't know, try this: - turn phone off - send message from online account to yourself - check thunderbird for the message. If you didn't receive it, then there may be a need to adjust your settings and you have have received error message. Post that error message and a screenshot of your incoming server settings - if you DID receive it, turn phone on and see if it receives it. If it does, check back to see if it's still in Thunderbird. If not, the phone may be set for POP, causing periodic loss of messages, depending on whether phone or PC is first to see message. - let us know.

I have had the same problem yesterday. My boss has replied to one of my email. I could see it on my smarthone Gmail app and i can read it on gmail on my computer. For an unknown reason, It has never been downloaded in thunderbird. I use thunderbid for many years, before smartphone did exist. If there was no gmail app on my smartphone, I would never now about this important mail. Server POP mail server

I suggest you do what I suggested to the other person; do some detective work. For example, turn phone off and shut thunderbird down. from online account, send two messages to yourself. Then, turn phone on and check for the messages. Then, start thunderbird and see if you receive the messages. If you do not, that is an indication that the phone is POP. I will also comment that, by having PC set for POP, you can never use phone and PC independently as mail receive on PC may not be visible on phone. My suggestion, if you use both phone and PC for email, is to set both to IMAP.