Here is what to tell people with winmail attachments.

Q: When I send mail to an Internet mailing list, its members complain that
my messages contain big binary attachments. What's happening? How can I
get rid of these?

A: Those attachments contain Exchange's rich text information, encoding
attributes of the message such as boldface, underlining, fonts, and
colors.  Exchange/Internet Mail puts these attributes into an
attachment so that they can appear to other Exchange users on the
Internet. The problem arises when people not using Exchange receive
these attachments: instead of seeing a formatted message, they see a
big chunk of UUENCODE data named WINMAIL.DAT if you're using UUENCODE,
or a section application/ms-tnef if you're using MIME, each full of
encoded data.

For communicating with users of other clients, Exchange contains an
option to suppress sending rich text information when mailing them. If
you double-click on an underlined recipient (the underline means that
Exchange has recognized the name, and associated an address with it) in
the To or Cc fields on the message form, and you're using the Internet
Mail provider, you'll see a check box labeled Send to this recipient in
Microsoft rich text format. Always clear this check box if you suspect
that your recipient isn't using Exchange. If this flag is clear for
every recipient on a message, Exchange/Internet Mail will strip the
rich text information when it sends it, eliminating the mysterious
binary attachments. Note however that if the message has multiple
recipients, and any one of them has the flag set, Exchange will include
the rich text attachments in the message, which all recipients will
receive, regardless of their particular flag setting.

Whether this flag initially appears set or not depends on the origin of
that underlined name. If you got the name by typing a literal
[log in to unmask] SMTP address, the check box will be clear by default.
If you got it by specifying a SMTP one-off - i.e.  by typing
[SMTP:[log in to unmask]] - it will be set by default. If you got it by
typing a name and letting the system pick it from your Personal Address
Book, it will have whatever value you have set on the name in your PAB.
Finally, here's the tricky part: if you got it by giving the reply
command in Exchange, Exchange guesses as to whether this should be set
or not, and when replying to Internet mailing lists, it always seems to
guess wrong. You can see this by reading a mailing list message in
Exchange, giving the reply command, then double-clicking on the
recipient you see in the To field of the note form, and there checking
the setting of the rich text check box.

You can work around this either by replacing the reply address with an
entry from your PAB that you know has rich text disabled, or else by
always manually clearing the check box as needed.