This is the point I'd usually setup a network bridge on my laptop and capture the data with Wireshark to see exactly what's going on.
As a process of elimination though and to isolate if it's the printer or the network you could try the following:
Download SendSMTP from here:
SendSMTP - Tool to send SMTP email via command-line parameters or interactively
Configure it like so (making sure username/password are correct):
settings 1.jpgsettings 2.jpg
See if you get an error message or check the log if it fails to see what the response you are getting from the server regarding the authentication failure.
That should help as the auth failure message on the MFD tends to be very vague but you can get better information looking at the actual message from the email server.
If on the other hand it works then at least you know the settings/account are ok and it is likely some issue with how the 455 is trying to authenticate.
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