-If the message already contains a \*(lqFrom:\*(rq field, then a
-\*(lqSender: user@local\*(rq field will be added as well. (An already
-existing \*(lqSender:\*(rq field is an error!)
+.PP
+A \*(lqFrom:\*(rq field is required for all outgoing messages. Multiple
+addresses are permitted in the \*(lqFrom:\*(rq field, but a \*(lqSender:\*(rq
+field is required in this case. Otherwise a \*(lqSender:\*(rq field
+is optional.
+.PP
+If a message with multiple \*(lqFrom:\*(rq
+addresses does
+.B NOT
+include a \*(lqSender:\*(rq field but does include an \*(lqEnvelope\-From:\*(rq
+field, the \*(lqEnvelope\-From:\*(rq field will be used to construct
+a \*(lqSender:\*(rq field.
+.PP
+When using SMTP for mail submission, the envelope\-from used for the SMTP
+transaction is derived from the \*(lqEnvelope\-From:\*(rq field.
+If no \*(lqEnvelope\-From:\*(rq field is present, the \*(lqSender:\*(rq
+field is used. If neither the \*(lqEnvelope\-From:\*(rq nor the
+\*(lqSender:\*(rq field is present, the \*(lqFrom:\*(rq field is used.
+When \*(lqEnvelope\-From:\*(rq appears in a message
+it will be removed from the final outgoing message.