PC/Outlook recipients complain about Mac Mail attaching blank untitledattachment*.htm files to emails

I have also seen in the past that Outlook often attaches a separate VCF contact file as a signature when observed from other email clients. This poses a real problem for some virus utilities that scan for email attachments.

The best thing to do is ask that people edit their email settings to send plain text and MIME attachments.

The other standards used in email are not universal.

You have no control over how the recipient's Outlook mail client treats your email attachments, including any hypertext that may be in a mail signature (if used).

The "*" symbol is known as a "wildcard" character, to represent any length group of characters both in Unix and in Windows.

If you are naming the file with an *, it will have unpredictable results.

Ah, well this happens if your email needed content that was HTML generated instead of plain text.

Well that's a problem on their end. Different email clients work differently when it comes to attachments.