Have you tried to set up another SMTP server at the account settings>Outgoing Server SMTP pane? I am not aware of that limit. My concern is that your new account may be attempting to share another account's SMTP server.

That is my problem - I am setting up new smtp servers in the identities when I create a new pop account, the pop account works fine but the new smtp server overwrites a previous one and any attempt to add the overwritten one as new overwrites the previous new one.

Managing SMTP servers in Identities is primarily designed to handle aliases, where multiple email addresses legitimately use the mother SMTP serverl, BUT, if you are setting up new email accounts that need their own email SMTP servers, then you need to make them separate accounts, not identities, and to have unique SMTP servers.

Identities are how I am seeing the problem. I set up a new email account, I got to the smtp settings and everything looks ok, I use the account to the point where I need to send email then the server responds "account name mismatch". I look at settings and find that the outgoing server has changed to a random other one, I edit the smtp server and it works fine. Until I use another random email to send and I get the same response and the I find the previously edited details on another account. It's a bit "whack a mole". I can't add a new mailbox for sending emails , no more smtp servers can be added.

There seems to be some confusion about the terminology. Please check this support article to understand what Identities are for Thunderbird. https://support.mozilla.org/en-US/kb/using-identities

Now, are you using one account with 60+ identities?

Or are you using 60+ accounts with only the default identity for each account?

In the latter case, are these really all different accounts, i.e. each one with a different user ID and password?

Are you aware of how different SMTP servers set up underneath Account Settings - Outgoing Server (SMTP) can be assigned to an email account? https://kb.mozillazine.org/Multiple_SMTP_servers_-_Thunderbird

I have 60+ accounts each with their own pop and smtp settings. I had no noticeable problem until I passed some limit at which point the smtp settings started overwriting each other as they were added. I think my solution is going to be to add forwards (on mail server) to a "notification" mailbox and use webmail for the least used mailboxes.

This may not be up to date information, but also I'm not aware of something has changed since. https://kb.mozillazine.org/Limits_-_Thunderbird#Accounts

You may have hit a bug though.

I'm not good enough at coding to go bug hunting so I'll go for the webmail workaround which will mean I can reduce the number of thunderbird accounts and see if the problem goes away. If it doesn't I've got corruption somewhere, if it does there's a limit that I can test. Thanks for the advice and questions, the questions helped me think it through. I'm willing to call this closed as it seems other people aren't having this problem.

I encourage you to at least attempt to set up the SMTP server at 'account settings>Outgoing Servers SMTP. You are seeing a different path when you attempt to use identities.

I have, this is exactly what I have said repeatedly. When setting up a new mailbox the smtp settings overwrite a previous smtp account. The identity does nothing more that show me that the smtp account is incorrect and editing the smtp server simply overwrites another one.

And it is the identities path itself that prevents your creating a new SMTP server. The path I specified doesn't restrict you.

If you have a set of steps that be confirmed (preferably in a new profile) to result in a failure, please create a bug report.

David I have followed "the path" Identities is not the issue, smtp server settings are. Wayne I am not submitting a bug report because I don't know enough about the problem and don't want to enter the settings for 60+ accounts in a new profile. I am declaring that I will reduce the number of mailboxes that thunderbird has to handle and I will reopen an issue when I can identify what the problem is. I expected other people to have run into this and it is not the case so I'm on my own. I am declaring this the solution - Thank you.

This is an interesting issue: Technically, the 'problem' is that one cannot create a new SMTP server definition from within the Identities pane. That is clear, as only 'edit' is available, and only to predefined SMTP servers. That can be confirmed immediately. However, I see no logical reason for a different SMTP server, since identities are aliases that are dependent on the core account, and therefore would use the existing SMTP server. That is probably why this issue rarely surfaces. So, is it a bug that a logically unneeded SMTP server cannot be created, or is it a bug that the setting of the SMTP server exists at all within the Identity pane? I would open a bug, but I'm hesitant to toss this conundrum to the developers to scratch their heads over.

In this case, an SMTP server could be defined via the Outgoing Server SMTP pane, and then linked by selecting from within the Identities pane.

I hope it's ok to add a bit extra, the problem was local corruption where several accounts had lost their smtp settings and used another at random. Manually adding new under Outgoing Server (SMTP) and then adjusting each mixed up account sorted it out.

I'm glad to you have it all working again. :)