Missing mail

Greetings-

I recently saw an email and read it, but didn't act upon it. (It was something to do with my business.)

I just went back to try and locate it and it isn't there; not only that, but no emails from either of the addresses I get mail from that company from are in my mailbox. They're not in Trash, Spam, and don't show up in a search.

Has anyone else had issues recently with mail going missing? I wouldn't have noticed it except it was an offer I was interested in.