This one has been puzzling me for a while!   The symptoms were as follows:

Running MailEnable under Windows/Plesk VPS

Redirect enabled for mail account to external address

All emails now bounce with:

#5.0.0 smtp;503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.> #SMTP#

Everything looked ok in the Plesk/Mailenable GUIs?

I eventually found the problem in the DOMAINS.TAB config file for mailenable (in my case this was under prog files\plesk\mail servers\mail enable\config)

The entry for the domain in question had a “.d” appended to the domain name.   It appears the config file hadn’t been updated properly when I’d enabled email for the domain.   As such, the domain in question wasn’t seen as local to the server by mail enable.

I manually removed the “.d” (always take a backup just in case) and the email started to flow!