I may need to write some C# code which sends email via IMAP instead of SMTP.

That means an SmtpClient implemenation like this is out and per this there is not a way do the IMAP stuff baked into C# itself. I'll have to use a third party library. Suggestions at the Stack Overflow thread include:

• AE.Net.Mail
• MailSystem.NET
• S22.Imap
• Mail.dll
• etc.