Before you add the second account, change the Account Name to a unique name in Tools/Account Settings, select the account in the left pane. TB defaults to using the email address as the Account Name, which causes the error message. Setting distinct Account Names for each account, and distinct Descriptions for the outgoing smtp servers, allows multiple accounts using the same servers, e.g. imap.gmail.com and smtp.gmail.com, and makes it easier to associate the correct smtp with each account in the Outgoing Server (SMTP) drop-down (in the same window as Account Name).

Hi and thanks @sfhowes for quick reply. Sounds promising, could you please elaborate.

Fact is that I already had changed the Account name as I'm used to do since same happening in other email clients (eg using the email address as Account name), so this is not the cause of this problem. I now also changed the SMTP name slightly so it will not be the same as for the new account. Sorry to say I'm still getting the "Incoming server already exist" warning and not able to add this new account. I have of course to keep the server names (eg imap.google.com and smtp.gmail.com) as I will otherwise not be able to access the gmail-based account. How have you done for making this function?

Thanks again in advance for all support.

Go into 'Offline' mode

• 'File' > 'Offline' > 'Work Offline'

then set up the account

• File > New > Exisitng Mail account.
• Enter the name, email address and password. click on Continue.

this time it cannot check server because all is offline.

• Click on 'Advanced Config' button to open Account Settings window and check/enter all settings and manually change the 'Account Name'.
• click on Ok.

Exit Thunderbird. Let background processes complete and restart Thunderbird and it should restart in Online mode.

Hi Toad-Hall and many thanks for instructions, these worked quite fine, with one caveat, it seems one must also disconnect WiFi/Ethernet since Thunderbird still fetches parameters from the ISPDB even when in off-line mode (the autoconfiguration seems still be online), done that I was able to create the account and restarted in on-line mode.

Server name was fine to set to imap.gmail.com, however not so when I set the account name to myname@gmail.com , since it complains that the server name (imap.gmail.com) AND the user name (myname@gmail.com) already exist, which is in fact correct since I'm already using it for my base gmail account. Thunderbird actually automatically renamed the user name (to the email address provided). Btw the OAuth2 started and I was able to log in etc. when using the "somethingelse@mydomain.com" as user name, but did of course fail in the end because the gmail user name was not the one supposed to be.

Any further idea what else to do ?

Many thanks for support.

Hi everyone, I have found the solution to the problem. Fact is, as above stated, that Thunderbird indeed only provides the possibility to use one and only one server account name and user name combo.

However, I found a slightly hidden setting on the Account Setting page named "Manage identities" (see attached screenshot) whereby one can add one or more additional email addressees (identities) connected to a specific server account name and user name. The name of the account remains the same , but you can actually choose the email address when composing emails. Thanks all for support! Stay safe (doctors say that Covid-19 risk is at minimum when sending emails/computing, alone of course!)