Both suck down the waste you don’t want. Both will also suck down the occasional thing you do want, like a misplaced ring, or an email from a friend who uses exclamation points like a kid eats candy and doesn’t know enough not to capitalize every other word.
And, from time to time, both need attention.
Here, though, is where the analogy ends. Unlike your kitchen sink, your desktop spam filter is almost certainly the second (or even third) system disposal for email. Unless you invariably use webmail, and religiously check its junk folder, I can almost guarantee you are missing emails that no one would ever think are spam.
The culprit here is Gmail.