I suspect you're just running into the fact that a period at the start of a line is used to terminate a mail.
From RFC 2821 section 4.5.2:
4.5.2 Transparency
Without some provision for data transparency, the character sequence "." ends the mail text and cannot be sent by the user. In general, users are not aware of such "forbidden" sequences. To allow all user composed text to be transmitted transparently, the following procedures are used:
- Before sending a line of mail text, the SMTP client checks the first character of the line. If it is a period, one additional period is inserted at the beginning of the line.
- When a line of mail text is received by the SMTP server, it checks the line. If the line is composed of a single period, it is treated as the end of mail indicator. If the first character is a period and there are other characters on the line, the first character is deleted.
In other words, I suspect when the mail is actually sent, the receiver will only see one dot. It's not clear why Outlook would do the wrong thing here though...