You could be the victim of excessive spam/virus control. Some groups I know
routinely delete ALL email containing attachments. Others use a point rating
system and delete everything above a certain point count. Still others
reject the email, but let the sender know.

If the former is what's happening, and if nobody is changing the settings
between two mailings, then any given email with an attachment sent to a
given address will either ALWAYS be deleted, or always accepted. But, from
email to email, you would get different results, depending on the deletion
criteria.

So... the question is... in your tests, has the identical email ever been
both accepted and "gone missing" when sent to the same recipient?