DOH, I just realized you were asking about ASP.NET and not PHPMaker - however I believe the points are still valid:
I don’t know about this specific issue but certain special characters like backslash, semi colon, etc. are commonly used in attacks on websites and so are often filtered out but more likely here it is the use of the backslash as an “escape character” in PHP. If you can change the password then get rid of that backslash and it is likely to work. If you can’t change it you might even try “\\” (two backslashes) which might get replaced with a single one (again those escape characters). I had to type 4 back-slashes just now to get those two to show up here in the forum.
Thanks for your reply - our SMTP server clearly complains about a wrong password.
Could you tell me please in which of the by ASP.Net Maker compiled files the SMTP password ends up. May be the ASP. Net Maker generating process interprets the \ or some other special characters as a control sign end writes the wrong password into that file.