(I hope this gets put into the right thread, as responding to a post doesn't seem obvious in this forum!)
Thanks for the response patrick_s, but NO, it doesn't seem to help my situation. Mail to my local domain is being put into the mail queue directory, and the sendmail log has status of "queued" for the mail that was received.
I'll check my aliases file to make sure my email addresses are pointing to my user accounts appropriately though.
Isn't the sendmail.cf file created by m4 (using the source outlined in these articles) specifying an Mlocal directive with the procmail program? If so, is procmail defaulting to just throwing mail into the users' home directory, rather then the Imap subdirectory that these articles are recommending?