Hmmm. I thought it was straightforward. I'll unpack it:

> Spam restrictions aren't generally applied by the government, and therefore don't fall under the constitution.

The "free speech" constitutional amendment stipulates that the government can't restrict speech. It doesn't apply to a mail service provider, which is free to reject whatever it likes.

> The law doesn't require anyone to listen to someone else's speech.

Your freedom to speak to me ends when I decide I don't want to listen to you. I have a right to not listen, and I have a right to reject spam.

> It is not a violation of anyone's rights to discard their emails unread using an automated filter.

I don't know how to say that more clearly; using a spam filter doesn't violate the US constitution. Email would be unuseable without spam filters.