HTML message body inserting random ! characters.

Some HTML emails were having what appeared to be a random exclamation point inserted. The problem only appears when HTML emails are edited through a WYSIWYG editor. From Googling, the issue appeared to be similar to this which occurs when HTML emails are longer than 998 characters. As most WYSIWYG editors remove line breaks and whitespace, this can be common.

A simple fix was to add a chunk_split to any mail message body passed through variable_email. This means only one patch is needed for all modules making use of variable_email.