(207) What does ‘Authentication failed’ mean?
Basically, ‘Authentication failed’, or similar, means that the email server of your email provider doesn’t accept the login (anymore).
+The first thing you should check whether you can still login via the website of the email provider. If you can’t login via the website of the email provider, you’ll need to contact the email provider for support. In any case, in that situation, no email app can login either.
The obvious reason is that the username (generally the email address, but not always) or the password is incorrect. There are other possible causes too, like connecting from a suspicious network (according to the email provider), connecting from abroad, or using a VPN. Some email providers are more critical than other email provider regarding accepting connections.
If you are using a VPN, please turn it off, or make an exception for FairEmail. With a VPN you share a network address with many other people, possibly not all behaving nicely. If an email server detects ‘unwanted’ behavior, like somebody trying to send spam messages, an email server often automatically blocks network addresses as a protection measure.
You can try to switch to Wi-Fi or mobile data because this will result in using another (local) network address. Turning flight mode on for a while and turning it back off will mostly result in another network address being assigned. So, this is worth trying as well.