it's got nothing at all to do with free speech of "freedom of
expression".
the OP is 100% entitled to say whatever he wants to say.
at the same time, i am 100% entitled to refuse to accept his email on
my servers for any reason whatsoever (including no reason at all). so
is anyone else. his right to free speech does not include the right to
force me or anyone else to listen.
my server, my rules. one of the rules i choose to use on my mail server
is that i do not want mail direct from dynamic IP addresses. like it or
not, nobody except me has any say at all in this.