I have seen dozens of accounts on Thunderbird; having three accounts would not be a problem to Thunderbird.

hello david, thanks, but it´s not about how many email accounts TB can handle. it´s about how many user accounts on one computer it can allow, if there is such a rule or limit. so for example it´s 3 users, all use this one computer. everytime one user logs in (to windows) at first TB is empty and the user can setup the connection to exchange. the first two users could do this but the third can not. thank you Andi

Thunderbird stores all of it's user data in the users %appdata% locations on windows , as you would know those are domain user dependent and access is not shared between users. So user 1 should not even have any data available for user 2 to even find. Unless you have installed Thunderbird using an administrator login and pointed the profile to somewhere other than the default locations for each user no user should have access the profile data of another.

So there could be a limit only in how many domain users can log into the computer (disk space for their remote and local data or memory if they are switching users.)

hello matt, correct, each user has it´s own area, can not see or access other user´s data, files etc. so thunderbird is installed once and every user can use it, like word or paint.

every user has enough space localy and on the servers...

meanwhile i found OWL for TB an i found TBSync for TB - this addons make sure you can connect to a exchange server or 365 enviroment.

TBsync is only working until TB 140 i think and OWL is not for free.

on top of that we have to think about to uninstall exchange and change to another system because like with windows 10, the support is ended with exchange server.

but the question is still, if there are addons like OWL to connect TB to exchange server, how was it possible for me to successfully connect 2 users to their exchange account with working emails and the third user can not connect. in the profile / connection settings if i try now to connect third user, there is a message i have to usw OWL

kind regards Andi

Sounds like a problem with your server - Thunderbird would introduce no concurrency issues.

As far as EWS, if you manage the server then you determine how long it remains operational.

Your choices with Thunderbird are to either use OWL, or (if you don't need calendar and address book support) you can use upcoming Thunderbird version 145. You can try 145 in beta https://support.mozilla.org/en-US/kb/thunderbird-beta

Thanks for all informations, we have a meeting this week about the support end of exchange server and what are our alternatives. maybe there is no reason to use TB anymore but still i´m wondering what the issue is here. i will try 145 beta also thank you