Since determining that Outlook was the problem, I've tried several things to get it to display correctly, including adding a "\r" and trying different Mime types, to no avail. Oddly, changing the contents of the string that displays directly before the timestamp sometimes causes the "\n" to display.

Further, I've tried different email address with different clients, and from the statistically insignificant number of clients I happen to have installed on this machine, it seems to be exclusively an Outlook issue.

Thank you to everyone who took the time to look at this.

Thanks,
cbeckley