Stick to the Text Formats
As far as receiving emails from others is concerned, the speed at which they are loaded is important. Email is supposed to be fast messages brought to us and in the even that some of them take some time to load, subscribers will not waste their time waiting for the page to load.
Indeed it is pretty much commendable to see html formats in our email boxes. But do remember, long loading emails to be read may become a total problem for most. One thing that anyone would hate are too much graphics and scripts which may eventually be considered as a virus which all of us fear the most.