A bug in .NET MailMessage: Invalid dot-stuffing when using quoted-printable encoding.
SMTP does not allow for leading '.' characters on a line, so they are to be escaped as ".." (dot-stuffing) for transfer (I think). Most of the time MailMessage does this for quoted-printable messages. I found an input that causes a leading '.' to sometimes be escaped as three dots. We also have encountered situations (not reproducible, yet) where we think '.' characters are removed from the message, probably due to errors in escaping.
- This appears to be fixed on machines with .NET 4.5 installed.
- It seems that the bug can be determined by a private reflection "check": Program.cs#L68
- I am not offered any .NET updates on my .NET 4.0 machines.
Issue: 781115