Gmail is one of the most reliable and reputable mail services. So it piqued our tech team when our billing division reported that Google seemed to think our server is a spam source.
We routinely keep track of all spam black lists, and were quite sure that the server reputation was not affected. The mails that were sent to other major providers like Yahoo, Hotmail, etc were not affected. Only Google was sending all our mails to the spam folder.
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The email header analysis
OK. So, it could be because of the recent HTML changes in the invoice, right? We needed to test that, and sent a plain text mail to our Gmail account. It still landed in the spam box. Hmm..
It was time for a more methodical analysis of mail headers.