Lost of attached files in email

When sending email from Outlook 2000 (version doesn't matter)
using linux SMTP server (Sendmail 8.7.3) from time to time happened
that addresee received email without attached files. Detail
checking of received mails says that Sendmail recognizes
some email part ("------") as end of MIME section and that correct
end of section was ignored. Remaining part of email was corrupted.