"The username or (app) password is incorrect"
- Use the app password, not your regular password. For Gmail it is the 16-letter code, for iCloud the code with hyphens.
- Copy the code again: one character too many or too few is enough to make it fail.
- With GMX and Zoho: is IMAP turned on in the settings of your email provider?
"This is your regular password"
Google then says an app password is needed. Create one (open app passwords) and paste that code instead of your password.
"Reconnect needed"
Signing in no longer works, usually because you changed your regular password: Google and Apple then revoke all app passwords. Create a new app password and click Reconnect on the account.
"The server cannot be reached"
Check the server address and the port under Server settings: receiving through 993 with SSL/TLS, sending through 587 with STARTTLS. Port 465 does not work from our server.
Something else
Email us at support@notabula.com. Tell us which email provider you use and which message you see. Never send your password or app password.