Quite possibly you answer your own question.

many users access to the email account simultaneously, so this feature of IMAP is required.

How many logins does the provider allow to the IMAP account? Given the many are all probably using more than one device, it may be you not getting updates because there are simply to many attempts to log into the same email account at the same time.

Hi,

thanks for your help.

No, this is not the point. I have 4-5 accounts, but I have the same issue using just 2 of them together. Then, using another Client, the issue is not present.

Additional info:

the status of the email is updated without a restart of Thunderbird ONLY for the last email of Inbox folder. I mean, if I change the status (read/unread) of the last mail, this is refreshed with a simple new download. All the others emails are refreshed just with a complete restart of the software.

To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Hi,

thanks again!

I tried both actions, but the result is the same. I tried also to configure a new PC with a new installation of Thunderbird and the situation is exactly the same. I guess there is something on IMAP management to be checked on Thinderbird, I really don't know how moving.

PS: I tried the last available version of Thinderbird and the Beta version too.

Updating: I solved (or I hope :)) the issue by setting the option "Maximum number of server connections to cache" to '1' in stead of '5' (default setting of Thunderbird). I really don't know how this affects to my server mail because the number of users is limited to 3-4, so I was guessing that the number of connection is lower than 4.

Anyway, thanks for help, hope that the problem is solved permantly.