Choice of a mail program depends somewhat on what system you're using.
If you want to contact me directly at brian@grot.starconn.com, maybe
I can help out off-line.
However, about doing a mail reader in MetaCard: maybe one could use
something like C-Client for handling the particulars of SMTP (and MIME,
if C-Client has been updated to support MIME) and then just build a nice
interface using MetaCard? C-Client was developed at the University of
Washington, and later redsigned at Stanford, to handle IMAP and SMTP
for various mail user interfaces. Hmmm...