MYDOMAIN does not refer to your corporate domain.  MYDOMAIN is the space-separated list of all possible host names your machine might be known as. In many cases the value of MYDOMAIN is the same as NODE, but for instance some machines running LISTSERV also run the enterprise's World Wide Web server, and may thus be identified in DNS as something like "WWW.MYCOMPANY.COM".  And of course we'll assume that NODE has been set above to something like LISTSERV.MYCOMPANY.COM.  Using this as an example you would set the MYDOMAIN setting to contain, in space-separated format, all of these names that mail might come in to LISTSERV under, with the NODE value coming first:

LISTSERV.MYCOMPANY.COM WWW.MYCOMPANY.COM POP.MYCOMPANY.COM


Alternately the same thing can be done by specifying

%NODE% WWW.MYCOMPANY.COM POP.MYCOMPANY.COM


This way if someone accidentally writes to LISTSERV@POP.MYCOMPANY.COM, LISTSERV will recognize POP.MYCOMPANY.COM as equivalent to LISTSERV.MYCOMPANY.COM and will process the mail as if it had been addressed to LISTSERV@LISTSERV.MYCOMPANY.COM.  Otherwise the mail will bounce as LISTSERV has no other way to know what other names the machine might be known by in DNS.