First let me say that all of my emails from our various projects get forwarded to Gmail where I pick them up using my mail program.
We have a client that was sending us emails regarding a site we were working on for him. We would receive a few of the emails he sent and the others would never be received. We were stumped as to why this was happening as the emails never made it to the in box OR the spam box. So we began some tests.
While on the phone with him, he resent an email that I had not previously received. I was logged in to my Gmail account and could see the email being received by Gmail and then it would suddenly disappear… not go in to the spam filter or any other mailbox, just vanish.
We talked about it for a while, did some testing and then realized that the emails all contained the word "pharmacy" because his new website venture was call pharmacykeyword.com (not the real domain, his projects all use trademarks he owns). Since "Pharmacy" was in the url and therefore referenced in the emails, Gmail was nuking them instantly.
The client changed the name of the site and all was well.