Yes, set the default user to pick these up. From the help:
Default user When distributing mail from a domain mailbox, MercuryD may encounter messages for whom it can find no local recipient; this will commonly happen if one of your users subscribes to a mailing list, since mailing lists usually do not indicate the actual recipient anywhere in the message headers. In cases such as this, MercuryD can be told to deliver the message to a specific, or default user. If you leave this field blank, MercuryD will discard any messages for which it can find no local delivery addresses. This field is only meaningful when you have told MercuryD to distribute mail (by leaving the Local user field blank).
Yes, set the default user to pick these up.  From the help:
Default user  When distributing mail from a domain mailbox, MercuryD may encounter messages for whom it can find no local recipient; this will commonly happen if one of your users subscribes to a mailing list, since mailing lists usually do not indicate the actual recipient anywhere in the message headers. In cases such as this, MercuryD can be told to deliver the message to a specific, or default user. If you leave this field blank, MercuryD will discard any messages for which it can find no local delivery addresses. This field is only meaningful when you have told MercuryD to distribute mail (by leaving the Local user field blank).