I also have my own domain name and give each entity its own customized email address so that I can better track where leaks occur. I can't speak for the OP, but in my case, the email addresses I create can consist of two words, a special character, and a number. There's no way that a spammer can guess this format out of the blue.

I have received spam at several of my custom email addresses, but thankfully, none yet to my FlyerTalk custom email address. I don't use my FT email address for login purposes; I always use my username.

Yes, there are other ways to get my FT email (such as via notifications that FT sends), but if/when I do receive spam to my FT email address, I'd be just as suspicious as the OP as to how it happened.