Messages contain both envelope (transport layer) addresses, used by the e-mail system but generally invisible to the user, and header (display) addresses, which are the addresses visible in the message as received by the user.
PMDF's addresses always come from either the message envelope (transport layer) or the message header. Addresses can be further categorized as being either From: addresses (more generally, addresses that point back at the message source) or To: addresses (generally addresses pointing towards the message destination). PMDF does alter its address processing somewhat depending on where the address appeared.
Transport layer To: addresses are rewritten in various formats depending both on what the channel table says the channel requires. That is, channel level address rewriting may involve special forms of channel definition that request special address handling. Transport layer To: addresses (envelope To: addresses) are the only addresses where channel level rewriting is applied. Header and envelope From: addresses are not affected by channel-level translation rules. Such channel block effects on addresses, discussed above in Section 2.3.2, occur after the regular address rewriting performed by domain rewriting rules.
Thus system name transformations that should not be performed in the message header may be placed in the channel table, while transformations to be applied to the header should appear as rewrite rules. Note that in many instances, the same effect may be had using rewrite rules and the $E and $B control sequences as described in Section 22.214.171.124.