Many public Wi-Fi networks block outgoing email to prevent spam. Sometimes you can workaround this by using another SMTP port. See the documentation of the provider for the usable port numbers.
If you are using a VPN, the VPN provider might block the connection because it is too aggressively trying to prevent spam. Similarly, the email server might block connections via a VPN because it was misused for sending spam. Some VPN providers have “cleaner” IP addresses than others, so switching to another VPN provider might be useful. Note that Google Fi is using a VPN too.
-The error ‘You must use stronger authentication such as AUTH or APOP to connect to this server’ can be fixed by enabling debug mode (last option in the miscellaneous settings tab page), and in the debug panel that appears enabling the APOP option.
+The error ‘You must use stronger authentication such as AUTH or APOP to connect to this server’ can be fixed by enabling debug mode (last option in the miscellaneous settings tab page), and in the debug panel that appears enabling the APOP option. After that, debug mode can be disabled again.
Send errors
SMTP servers can reject messages for a variety of reasons. Too large messages and triggering the spam filter of an email server are the most common reasons.