mail server settings not being saved

iMac High Sierra 10.13.4

I have three Mail accounts, on two of them I had changed passwords and I since then I cannot send e-mails because the server settings is not being saved. The third account is OK because there I had saved the password couple of months before and I guess that whatever bug causes this problem wasn't present at that time.

So what happens is that when I send an email, I get an error 'authentication failed'. So I go to Mail preferences, Server Setting, fill in all the necessary data, press 'save', I get a spinning wheel with a "Verifying Server Settings" msg. The message disappears and about five seconds later the outgoing server's name spontaneously changes into

'none'. So I select my outgoing server and up pops the old setting. I tried quitting Mail, re-booting, etc. but when I eventually go to check the setting it's ale=ways the old information. Of course, when I try to send an e-mail, I get 'authentication failed' again.

So eventually I try to send an email during the few seconds before the server goes to 'none', and the email is sent with no problem!! Then the server again gets mysteriously unselected (and the subsequent emails fail). So it looks like when I press 'save', the settings get successfully verified and put where it should be but within few seconds it reverts to the old information again.

I think the server name morphing into 'none' maybe happens only since the last OSX version update. Before it was "Unable to verify user name or password".

Anyone any ideas what is the cause and/or the cure?