delete smtpserver causes mail to freeze cannot reboot

First, as many have already stated, it is impossible to delete an SMTP server using the "Edit smtp server list" option when accessed through Mail/preferences/[select email account].

But let us say that you attempt to send and email using a properly configured account and Mail opens a dialog telling you that smtp server cannot be used (even though the connection doctor suggests that both the imap and smtp connections are up).

This dialogue has the option also to "Edit smtp server list". If you open the server list and select a smtp server to delete using the minus ➖ button, the dialogue will freeze. If you open "Force Quit", mail is still responding. But I've waited up to 30 minutes for Mail to return.

Now, if you choose to reboot your computer (restart from the apple menu), the process will be halted by Mail. Choose this again several more times (the exact number appears to vary) and each time "something" being displayed by Mail (the dialogue box; the email that originally would not send and demanded you choose a different smtp server; or the Message Viewer itself) will disappear and eventually -- without choosing to Force Quit Mail -- the computer will restart.

When it does, opening Mail, you will find that the smtpserver you tried to delete is still present. With luck though, you will be able to send email from the account that was having trouble.