I'd like $10 for every time I've seen this problem.
Don't worry, you've probably done all the right things and it's bad software on your server that's causing the problem.
Basically, the "official standard" (called an RFC) says that lines in the body of an email should be terminated with carriage-return + line-feed, but that there doesn't actually have to be any lines (it can be one long line). Line-feeds alone don't terminate a line.
Typically, email creation software will terminate lines with a single line-feed and that's perfectly OK - it's just that they are not "lines" in the official sense.
But, some server software sees the single line-feeds and treats them as proper lines and then somehow gets everything wrong and decides to double-space all lines!
Confused? Don't worry....you don't need to understand that (I'm not sure I understand what the server software thinks it's doing, either).
You're lucky you're using version 2.15. The solution is to look for the configuration setting in FormMail called "BODY_LF".
You'll see the following two lines:
Simply change them to this:
//define("BODY_LF","\n"); // the old default: just LF
define("BODY_LF","\r\n"); // the new default: use this for CR+LF
That should resolve the problem.
define("BODY_LF","\n"); // the old default: just LF
//define("BODY_LF","\r\n"); // the new default: use this for CR+LF
If not, we may have to do more investigation.