Being a monolith, it can do lots of really nasty things to spammers and that functionality is easy to set up due to the relative simplicity of the Exim configur

Being a monolith, it can do lots of really nasty things to spammers and that functionality is easy to set up due to the relative simplicity of the Exim configuration. It also can do things that other MTAs can't do at all, again due to it being a monolith. Postfix for example is made out of a string of independent programs that pass mail to one another. Once a program has an email that program can only deal with the email itself or pass it along to the next program. That limits what can be done with that email and makes the configuration harder to understand.