There are quite a few posts on the internet, I found, that have to do with the cause and the fix for an error sending email in DotNet:

An invalid character was found in the mail header: ';'

The problem for those was due to problems with an attachment. I got this error, and I had no attachments whatsoever in my email routine.

My application is a Winforms application, and in my email routine, I rely on application settings for the ‘To’ MailAddress. As it turns out, someone had decided they wanted to be emailed along with the main person set in the application setting. Therefore, in the application config file, they added their own email address, with both of them separated by a semi-colon. This is what caused the problem on our end. Once the semi-colon and the secondary name were removed, everything ran smoothly