I have had some problems recently adding another email account that I own to my gmail account as a ‘send as’ account. I have the option of using GMail SMTP servers but it adds the annoying ‘Sent on behalf of…” to the messages and I already have a postfix server setup for this account.

I would try everything offered. TLS, SSL, Unsecured, All different ports that I know I had open and working for years. I kept getting this error:

Your other email provider is responding too slowly. Please try again later, or contact the administrator of your other domain for further information.

Which is complete crap because I’m tailing the postfix log file and it never even hit me.

Well, as a test instead of giving GMail the DNS hostname of my postfix server I gave it the direct IP address and it just worked instantly. It seems the error is related to DNS. Not sure why, everything works great elsewhere.

Working for me now. Hope it helps somebody else.