The offline status in Mail indicates a mail server or network problem. It’s not related to SpamSieve (which doesn’t use the network or talk to your mail server). You can temporarily disable SpamSieve for troubleshooting purposes to see that it doesn’t affect this.
What happens when you try to add the account back?
It gives a “Unable to verify username or password” message when trying to set it up, even though I can log directly into the website with the same details.
I’ve got a 2021, M1 iMac running OS 13.4
After installing SpamSieve I found an “Unknown” entry in Internet Accounts. It has an iCal icon and I get a “The operation couldn’t be completed. (com.apple.extensionKit.errorDomain error 15.)” message when I click on it.
I suspect that’s probably a corrupted version of the account I’m trying to re-add.
Often, that actually indicates that the network connection is blocked or that the server isn’t responding, not that the username or password is wrong.
I’m not sure what that be. It seems unlikely that a mail account would have an iCal icon or use ExtensionKit. If all else fails, you could try removing the Accounts4.sqlite database in case it’s corrupted.