We noticed a little while ago that some ISP email servers were stripping CSS from HTML email, because the CSS code had (.) periods at the beginning of their lines. The workaround was to insert a space at the beginning of any line beginning with a period. But we recently came across an HTML email campaign that was breaking completely because of periods in its plain-text alternative (not in its CSS, and not even in the HTML content).
The problem turned out to be a bunch of periods at the beginning of a line in the plain text alternative version of the sender’s email. For those of you that don’t know, MailChimp sends email in multipart-alternative format, so that you can send an HTML email with an embedded plain-text alternative message that displays for people who can’t view HTML. The plain-text alternative is almost never even seen by most people who use modern email applications.
The sender wrote a line in his plain-text alternative message that started with:
…And a correction:
Those (.) periods at the beginning of that line apparently caused an email server (one of his test accounts) to perceive it as "end of message." The result was that everything after those periods was deleted. Even if you could view HTML email, it wasn’t displaying, because the only thing that made it past the email server was the portion of the plain-text email before those (.) periods.
The workaround (insert a space at the beginning, so that the line doesn’t start with a period) fixed the problem.
So be sure not to start any lines with periods. CSS code is the most obvious place to look, but it can happen in the body of your message, and even in the plain-text alternative message.