Sending issues using HTML

[Gnomey]

Hi Guys.
Firstly, I am a computer idiot so maybe this is something I have set up wrong.
I can send emails in HTML no problems.
I can receive emails in HTML no problems
BUT
If I try to reply to and email it will just keep attempting to send and then it will time out.
I can reply if I turn off the HTML though.

[alec]

It might be wordfence appliance or mod_security on the http server.