Sending email with a custom envelope sender (VERP)
I'm writing a server that will send email to many recipients on behalf of my client.

The email must come from the client's email address (client@example.org), but I want to automatically handle bounces via [VERP](http://en.wikipedia.org/wiki/Variable_envelope_return_path).  Basically, the email `From:` header will be client@example.org, but the SMTP envelope sender (`MAIL FROM`) will be *unique-email-id*@my-email-service.example.com.

I've already built a multi-threaded sending engine that uses the built-in `System.Net.Mail.SmtpClient` to actually speak SMTP with the recipient's MX server.

Unfortunately, `SmtpClient` does not allow you to specify the envelope sender &ndash; it just uses the `From:` address.

I need an alternative that allows me to specify the SMTP `MAIL FROM`.  Preferably, something that takes little work to drop in and replace `SmtpClient`.

Thus far, everything I've looked at is an entire [email](http://mailsystem.codeplex.com/) [suite](http://www.lumisoft.ee/lsWWW/ENG/Products/Mail_Server/mail_index_eng.aspx?type=info) (SMTP/POP3/IMAP/kitchen sink).  What lightweight SMTP libraries are available?
Sending email with a custom envelope sender (VERP)
===

I'm writing a server that will send email to many recipients on behalf of my client.

The email must come from the client's email address (client@example.org), but I want to automatically handle bounces via [VERP](http://en.wikipedia.org/wiki/Variable_envelope_return_path).  Basically, the email `From:` header will be client@example.org, but the SMTP envelope sender (`MAIL FROM`) will be *unique-email-id*@my-email-service.example.com.

I've already built a multi-threaded sending engine that uses the built-in `System.Net.Mail.SmtpClient` to actually speak SMTP with the recipient's MX server.

Unfortunately, `SmtpClient` does not allow you to specify the envelope sender &ndash; it just uses the `From:` address.

I need an alternative that allows me to specify the SMTP `MAIL FROM`.  Preferably, something that takes little work to drop in and replace `SmtpClient`.

Thus far, everything I've looked at is an entire [email](http://mailsystem.codeplex.com/) [suite](http://www.lumisoft.ee/lsWWW/ENG/Products/Mail_Server/mail_index_eng.aspx?type=info) (SMTP/POP3/IMAP/kitchen sink).  What lightweight SMTP libraries are available?