Question

I am using System.Net.Mail to to send mail. I do not know the type/version of the SMTP relay that it will connect to.

Some errors will result in no email being sent (e.g. no addresses or invalid from address) whilst others errors will still result in an email being sent.

E.g. Send To : [e-mail] CC: [e-mail] and [e-mail] may result in an error of

The server response was: 550 #5.1.0 Address rejected [e-mail].

But the email still appears to be delivered to Bob and Fred.

Is there any reference to which error codes will still result in an email being delivered or any programmatic way of determining this?

Answer 1

This is the way that SMTP operates, as described in the RFC.
Failure to deliver to one recipient does not affect delivery to other recipients.

Generically, codes starting with 4 and 5 are failures, codes starting with 2 are success codes – see RFC821 section “4.2.2. NUMERIC ORDER LIST OF REPLY CODES”.

“Email being delivered” is hard to define. Email being accepted for delivery by the SMTP server is defined by the SMTP protocol (and the server will signal acceptance to deliver a message) but this server might just relay the message to another server, or operate in a smart host configuration where it just accepts messages and passes them on to the smart host – delivery is another thing and it’s usually associated with local delivery (LMTP).

Some mail clients work around this problem of not being able to tell if a message has been delivered by implementing the dreaded read receipts – but this implementation is entirely on the client side and AFAIK it has nothing to to with SMTP.