Resolving 'The specified string is not in the form required for a subject.'

Also there is a limit of 168 characters so you should check for that too.

UPDATE: sorry this is complete bullshit :) It must have been a line break in my case.

I haven't personally tried it, but according to this, you only need:

subject = subject.Replace('\r', ' ').Replace('\n', ' ');

or something equivalent.

Internally, the MailMessage class will check the subject with:

if (value != null && MailBnfHelper.HasCROrLF(value)) 
{
   throw new ArgumentException(SR.GetString(SR.MailSubjectInvalidFormat));
}

So the only limitation (for now) happens to be the presence of CR or LF.

For VB.NET

subject = subject.Replace(vbNewLine, "")

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