outlook user's email to an Earthlink totalaccess 2003 user shows up as .dat

[hakalugi]

outlook user's email to an Earthlink totalaccess 2003 user shows up as
..dat...

the sender is on outlook XP and has already tried changing the mail
formatting destined for this usert to be sent in 'text only' mode. (by
right clicking the email address for the recipient, and overriding
'let outlook decide', etc)

why would an (i assume) outlook .dat file show up if it's being sent
in plain text?

any ideas?

thanks.