Mac Mail forgets my email password every few days. How to fix it?

For years, and it still is true, I've had the pattern that Mail claims to have forgot the password but it's actually reacting to some other problem. If you hit the (Cancel) button on the "enter new password" dialog, you should find that it continues to work just fine.

At one time, this syndrome was clearly associated with network problems talking to the mail server (Microsoft Exchange, as it happened); I could tell this by correlating messages in the Console log. More recently, I haven't always been able to identify the underlying problem, but the trick of canceling the password prompt still seems to work all the time, for me.

One thing to try is opening Keychain Access and manually deleting all Gmail related items, then restarting Mail and re-authenticating. I've noticed sometimes that older Gmail passwords tend to stick around in there.

I have seen this error message on accounts where I know Mail (or how it interacts with the keychain) isn't the problem. The problem is the alert is wrong but Mail isn't smart enough to tell the difference between a network/server problem and a password problem.

What I presume is happening is the Mac can't get a log in to the server and prompts you to check the password when the problem is entirely on the server / network end. No amount of re-entering the password into the keychain will fix this sort of error - and rather than go through the steps to re-enter the password, you can check online by logging into webmail with the stored password.

If that works, you know the keychain and Mac are fine and instead look at the network or mail server as the cause of the temporary failure that mail is blaming incorrectly on a password issue.