Beginning with V8.7 sendmail, notification of successful, deferred, or failed delivery is now done by using DSN (see RFC1891). The T= delivery agent equate provides three pieces of required information to DSN. The pieces are separated by the slash character:

T=mta-type/addr-type/diag-type  

The first piece, the mta-type, is later supplied to the Reporting-MTA: DSN header as its first argument:

Reporting-MTA: dns; here.us.edu
                
               mta-type here

The second piece, the addr-type, is later supplied to the Final-Recipient: DSN header as its first argument:

Final-Recipient: rfc822; badname@here.us.edu
                  
                 addr-type here

The third piece, the diag-type, is later supplied to the Diagnostic-Code: DSN header as its first argument:

Diagnostic-Code: smtp; 550 <badname@here.us.edu>... User unknown
                  
                 diag-type here

If the P= for a delivery agent is [IPC], an undeclared mta-type defaults to dns, an undeclared addr-type to rfc822, and an undeclared diag-type to smtp. For any other P= the default for an undeclared entry is NULL.

In configuring with the m4 technique, the declarations of the T= delivery agent equates are:

T=X-Phone/X-FAX/X-Unix       fax
T=DNS/RFC822/X-Unix          cyrus, cyrusv2, local, ph, pop, procmail, qpage
T=DNS/RFC822/SMTP            all SMTP and LMTP agents
T=X-Usenet/X-Usenet/X-Unix   Usenet
T=X-UUCP/X-UUCP/X-Unix       all UUCP agents
T=DNS/X-DECnet/X-Unix        mail11
T=X-Unix/X-Unix/X-Unix       prog

Other than for the local delivery agent, you cannot change these T= defaults. If the need arises, you can, however, copy an existing delivery agent definition and then modify it as outlined in Section 20.3.2.