It is a constant complaint, that mail just disappears from the thunderbird folders. I have used the program for years without any issue but one. So why has my experience been so different! the answer to that is I understand the email process and go looking for the obvious traps that other don't know to look for.
Todays I am going to look at the things that make mail disappear in IMAP accounts.
The first of these is inherently linked to the fundamental difference between IMAP and POP mail and for those that are used to POP, it is a real trap. Most Mail providers, (excluding the free ones, ironic huh), only offer limited mail storage on the server and instead institute an aging process which will automatically delete mails when they reach a certain time from the date delivered. If your mail is consistently disappearing if it is 30 days old, this is most likely the cause. The time frame may vary, but it will be consistent accross all of your folders.
It has become common for people to access their mail on more than one device often a mobile phone and a computer. Mobile phones almost ALL use the IMAP protocol to get mail. This is partially as people rarely want to do more email on their phone than they have to, and using IMAP leaves a copy on the server to be downloaded onto the computer, but mainly because it provides synchronization of mail between devices. This synchronization is all good, until you throw a mail client into the mix that is configured as POP. What happens is that POP deletes the mail off the server as soon as it downloads it, or in the case of Thunderbird 7 days latter. The net result is that the mail inexplicably disappears from the other devices as soon as it is deleted by the POP client. Welcome to the wonderful world of synchronized mail on devices.
The third of the I lost my mail is simply not understanding the synchronized nature of IMAP. The phone memory is getting full so delete some mail to free space. I have a copy on the desktop anyway. Not any more you don't. Deleting it on the phone with automatically lead to it being deleted on the server and the other connected devices as soon as they connect to the server.