You shouldn't post in HTML (Hyper Text Mark-Up Language).
Three reasons are:-

1. Such files are considered as binaries and not just plain text.

2. Often times html can get quite excessive and elaborate as it offers more text flavour and freedom. So what can be simply done is now way over board and taking up valuable server space.

3. Some readers use certain software that doesn't understand html. So all they see is none sensible rubbish.

Remeber, set your email for the mailing list to send in Plain Text