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@pippo_franco: if you rightclick on any of your imap's folders, select "properties", and then, in the small thunderbird-native popup window, go to the most-right tab "quota" (or perhaps "contingent" or similar)...: What does the small thunderbird's popup window show there? Is it the server's disk quota (again), or your mailbox' quota?

If it's the server's quota too, there, then you discovered a bug that needs to be discussed with the helpdesk/admin of the server. Because any tool that uses imap-protocol's way of retrieving the capacity and its utilization (which is given to it by the imap server) would get the same wrong data.

@ options button / TB60: enable tb's menu bar and select: Extras >> add-on-SETTINGS >> IMAP Quota (Free Space)

Rating: Currently/Still using TB69 with IMAP Quota (Free Space) v1.2, which is quite unstable. Configured limits are 0/70/90, so there should always be data shown. Sometimes it indeed shows the small bar with data, sometimes not, sometimes on the 2nd/third/etc. attempt. Sometimes situation improves temporarily after rebooting TB.

Good: Config options, display fashion (small bar with distinguished colors and percentual values), if bar is clicked then it opens TB's quota tab where the most detailed data is available.