Tip: Searching the Gmail spam folder
Customers who forward their mail to Gmail occasionally tell us that they can’t find a message they know someone sent them, even when they’ve searched Gmail for it.
These messages are often eventually found in the “Spam” or “Trash” folders of Gmail. What’s surprising is that by default, Gmail search doesn’t look in these folders at all, so people are (quite reasonably) sure it’s not there.
When you search Gmail for a message because you’re hoping to find it in an obscure folder, make sure you click the drop-down menu and choose the “Mail & Spam & Trash” option:
You have to choose that option: even choosing “All Mail” wouldn’t find messages in the Spam or Trash folders.