Having your ASP.NET error handling routine, which sends you emails when an error occurs on your site, itself fail is annoying. Especially when you think you’ve made the code robust enough. Anyway the error handler for one site I work on was failing with “ArgumentException: The specified string is not in the form required for a subject“.
So what is exactly “the form required for a subject”? Googling for this error message returns a lot of junk and misinformed forum posts. It turns out that setting the Subject on a System.Net.Mail.Message internally calls MailBnfHelper.HasCROrLF (thank you Reflector) which does exactly what it says on the tin. Therefore one forum poster’s solution of subject.Replace("rn", " ") isn’t going to work when your have either a carriage return or line feed in there.
So, obviously, the solution is:
message.Subject = subject.Replace('r', ' ').Replace('n', ' ');
Personally, I think that the MailMessage should to this for you or at least Microsoft should document what actually constitutes a “form required for a subject” in MSDN or, even better, in the actual error message itself!
Duncan Smart's Blog 
Thank you for taking the time to post the solution. If everyone did this, it would be much easier to do what already should have been done.
Thanks Duncan very helpful and well written post. I was able to fix the problem I was having
I ran into the same problem today. Fortunately for me your web site was the first one that I looked at. Thanks so much for taking the time to post this.
Good post!
Actually, it’s better also to check subject on empty string to prevent ArgumentException on ‘replace’ function.
Thank you for taking the time to post the solution. If everyone did this, it would be much easier to do what already should have been done.
Very helpful post, Thank you!
It saved me a lot of time searching for it. Found on 3rd posting I read.
I recently came across this problem and I can validate that what you have posted is exactly what is happening.
This leads to a big problem though. According to RFC 2047 (http://www.ietf.org/rfc/rfc2047.txt, section 2. Syntax of encoded-words), if a subject line is longer than 75 characters, it should be broken up into multiple encoded words. Each encoded word should be separated by CRLF SPACE. If this is not done, then it is possible that the subject line will be clipped.
How are we supposed to get around this limitation and conform to the specification? I can’t wrap my head around this. Any help would be greatly appreciated!
Thank you for you post. It was very accurate and the right solution to the problem.
Big thanks for sharing this, you made my day 🙂
Thanks! You saved me alot of time! 🙂
Thank U Dunkan,
It took me more than 6 hrs to land on u’r page for solutions, it’s really helpful.
thanks
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I found a very similar write-up to this one on a different blog site, but I like your opinion on it a lot better. Thanks for the added perspective.
It helped me a lot, Thank you
Huh. This saved my day. Thanx!!!
Thanks man! You saved my day!
Nice, but how do I insert a newline?
It’s not allowed as per the SMTP spec, so you shouldn’t!
Ok, Thanks a lot!
Thank you! Annoying none of this is mentioned in the description of MailMessage.Subject on MSDN.
Very helpful post, Thank you so much!