I usually just do "marketing@sitename" for whatever site it is. Let marketing sort out their own spam. Sometimes I'll just do "postmaster@sitename" or "admin@sitename" but marketing is usually my go to.

Many forms simply do the most basic regular expression possible of looking for alphanumberic characters with an @ sign before a period somewhere in the mix, so you can enter entirely nonexistent domains, but some are a little more in-depth