I created a new account on a web site and used my Gmail address (actually I used an alias that forwarded to my Gmail account) but I never received the e-mail. I logged in to my old Yahoo Mail account and sent myself a test e-mail, but that worked fine, proving that the alias was working.
I double-checked the e-mail address for the new account to verify that it was the same as the one I had tested. They were identical.
I logged in to my mail server to look at the logs and saw that the e-mail was being received and sent correctly. I wondered if my mail server was somehow messed up, but that didn’t make much sense and it was clearly forwarding the outgoing e-mail to Google’s mail servers.
I thought the e-mail might have gotten queued up in their servers, but my test e-mail showed up seconds after I sent it. It just didn’t make any sense the more I thought about it.
I just couldn’t figure out why I wasn’t getting the e-mail.
Then I figured it out.
Gmail had considered the e-mail Spam and filtered it out of my Inbox, and since there’s no indication that there are any e-mails in the Spam folder, I had no idea it was there.
The executive summary: If you are expecting an e-mail to arrive in your Gmail account and it never arrives, check the Spam folder before you do anything else. It could save you a lot of time.