The MTA often receives addresses that do not contain domain names from misconfigured or incompliant mailers and SMTP clients. The MTA attempts to make such addresses legal before allowing them to pass further. The MTA does this by appending a domain name to the address (for example, appends @siroe.com to mrochek).
For envelope To: addresses missing a domain name, the MTA always assumes that the local host name should be appended. However for other addresses, such as From: addresses, in the case of the MTA SMTP server there are at least two reasonable choices for the domain name: the local MTA host name and the remote host name reported by the client SMTP. Or in some cases, there may be yet a third reasonable choice—a particular domain name to add to messages coming in that channel. Now, either of these two first choices are likely to be correct as both may occur operationally with some frequency. The use of the remote host’s domain name is appropriate when dealing with improperly configured SMTP clients. The use of the local host’s domain name may be appropriate when dealing with a lightweight remote mail client such as a POP or IMAP client that uses SMTP to post messages. Or if lightweight remote mail clients such as POP or IMAP, clients should have their own specific domain name which is not that of the local host. Then add that specific other domain name may be appropriate. The best that the MTA can do is to allow the choice to be made on a channel by channel basis.
The noremotehost channel keyword specifies that the local host’s name should be used. The keyword noremotehost is the default.
The defaulthost channel keyword is used to specify a particular host name to append to incoming bare user id’s. It must be followed by the domain name to use in completing addresses (in envelope From: and in headers) that come into that channel. (In the case of submit channels, the defaulthost keyword’s first argument also affects bare envelope To: addresses.) An optional second domain name (that has at least one period in it) may be specified to use in completing envelope To: addresses. nodefaulthost is the default.
The switchchannel keyword as described, in the preceding section, 18.104.22.168 Alternate Channels for Incoming Mail (Switch Channels) associate incoming SMTP connections with a particular channel. This facility can be used to group remote mail clients on a channel where they can receive proper treatment. Alternatively, it is simpler to deploy standards-compliant remote mail clients (even if a multitude of noncompliant clients are in use) rather than attempting to fix the network-wide problem on your MTA hosts.