Delivery agents that speak SMTP require that any line of the message that begins with a dot have that dot doubled. Delivery agents speak SMTP or LMTP when the $u is missing from the A= delivery agent equate. For example:

Mether, P=[ICP], F=msDFMuCX, S=11, R=21, A=TCP $h

An example of a file that contains leading dots is a troff(1) source file:

.\" Show example
.Ps
Mether, P=[ICP], F=msDFMuCX, S=11, R=21, A=TCP $h
.Pe

In this example, three lines begin with a leading dot, but the F=X delivery agent flag causes sendmail to transmit the message as:

.." Show example
..Ps
Mether, P=[ICP], F=msDFMuCX, S=11, R=21, A=TCP $h
..Pe

The extra leading dot is automatically restored to a single dot at the receiving end.

This F=X delivery agent flag should be used only with delivery agents that speak SMTP or LMTP. It should not be used with other delivery agents because they will not know to strip the extra dots.

Note that declaring the local_lmtp FEATURE causes this F=X delivery agent flag to be automatically included for the local delivery agent (as well as F=z, F=z).