Here is my trick for this: I use an individual email address for each signup I do, in the form of <signupdomain.com>@mydomain.com. By that way, when I get spam,

Here is my trick for this: I use an individual email address for each signup I do, in the form of <signupdomain.com>@mydomain.com. By that way, when I get spam, I don't only know the spams source, but I can block it with only one click by blacklisting the specific receiver email address.