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Is your mail server in an RBL?
What address are the emails being sent from? as 550 5.7.1 can be relay denied.
Considering the error is thrown after the end of the data command, the content of this mail be be seen as legitimate spam by gmail.
I would recommend testing the SMTP transaction using telnet and the exact from address used by your server.
Regarrrrrrrrrds, pyrate ;0