In theory, any and all unencrypted data can be seen by anyone between you and the point of origin. Your ISP may, or may not, decide to block data from crossing it's network based on the headers, the protocol, the source, the destination, anything.
Most ISPs outside of the "internet black holes
" don't do it, with some notable exceptions1, 2
The information in e-mail headers is often used to catch spam. It would be trivial to filter for other things. The only real way to bypass that kind of filtering is to switch ISPs. Web-based e-mail would be a bit different, but the basics would be pretty much the same.
If that's what's going on with your ISP and you decide to switch, be sure to tell them why. Let them know that it's not OK to censor the internet.