Beginning with V8.12, sendmail supports the DELIVERBY SMTP extension (defined by RFC2825). If the DeliverByMin option (DeliverByMin) was defined with a positive value, a BY= equate can follow the SMTP MAIL FROM command for inbound email. The BY= equate defines the window of time during which the message should be delivered. That equate's time value can optionally be followed by a flag that states what to do upon delivery (an r flag), or upon delivery failure (an n flag). There can also be a flag that advises sendmail to trace delivery (a v flag). The BY= information is saved in the qf file on an ! line:

!flag time

Here, the time is the time specified by the By= and the flag is a one-byte integer that encodes a value of 1 (notify the sender upon delivery), or 2 (return the message if it cannot be delivered in time). Additionally, the flag can be OR'd with a 0x10 (trace the delivery).

This ! line appears for inbound mail only, and appears only if you have configured your mc file to define the DeliverByMin option with a positive value.