I encounter this pervasive and nagging bug in Evolution Mail's strange relationship with its co-GNOME app called Seahorse (although this app goes by the name of Password And Keys now in GNOME 3.26). I have a secondary email account and I want to use it for sending mail (smtp) temporarily (just in this case). So I finished composing my email and clicked send. Then it asked me for my smtp password and I thought that was odd because I have been receiving my email in the inbox for this account. It's unlikely a user would have a different password for login and another for sending email. But I found my password from my password manager and gave it the correct one ( copy and paste). And here it began, the Mail Authentication Failure window from Evolution continually asking for the password. I checked Seahorse for the password by entering a filter in the search bar. It has my password and it is the correct password. So what is happening here?
I found this same problem in the Ubuntu forum and Ask Ubuntu forum where there were several suggestions of deleting the email account in Evolution, or deleting the entry for the email account tokens in Seahorse. There was even a couple of patches introduced back in 2014 so it should be included in the latest version of Evolution Mail and Seahorse. But again here we are.
My last desperate attempt at fixing my problem comes through the GNOME Online Accounts where it is now just a tab in Settings.
So here goes. My first step was to close both Seahorse and Evolution. Then I opened Evolution, to verify the problem is still there. I might have logout too. But I don't see the point.
I disabled the account. In my case I have only one account nagging me. So I disable it by going to ALT+E(dit), A(ccount) and choosing the account.
I opened the GNOME Online Accounts and choose the available services as you can see above. This is when I remember that I didn't create an account here for GOA. Tip: if you have an account in Evolution and it didn't come through GOA you might encounter this problem.
That's it. Once the account is created in GOA, it appears in Evolution. And no nagging.