HTML mail cant be read

[Guest]

We've had reports that our clients can't 'read' the email
we've sent in HTML format.

I know Outlook 97 can't receive HMTL email is anyone aware
of other mail clients that struggle with HTML or block it
by default.

We've eliminated the possibility that a mail server is
blocking the HTML message as spam

I posted this on google earlier not really knowing what I
was doing so apologies if this has shown up twice.

Thanks in advance for any help.