I have found these error messages. While trying to add another email address. Please note that it is working very well with (email@example.com) but when I am trying to add another email address (firstname.lastname@example.org) that is provided by a different server I got these errors.
Unable to login. Check SMTP settings.
Failed to connect to smtp..com
However, if I disable SMTP & only keep POP3 enabled it says
Email updated successfully
It seems some SMTP issue with the email address.