When sendmail bounces a mail message, it does so
using DSN (T=). The type of address and the
actual address of the final original recipient are reported in the
bounce message:
Final-Recipient: RFC822; nosuchuser@site.com
This Final-Recipient line shows the type of
address (here RFC822) and the actual address (here
nosuchuser@site.com) of the final recipient.
Beginning with V8.10, sendmail composes this
address expression only once (when the message is queued) and stores
it in case the message bounces. The text following the
Final-Recipient: is stored in the
r line in the qf file, and
looks like this:
rRFC822; nosuchuser@site.com
Note that sendmail performs no checks on the
text following the r. This means that invalid DSN
information placed there will become the text that follows the
Final-Recipient: in the bounce message.
