This attendee reply is outdated. It will be ignored.

When I send an invite to a clients and they accept the invite I get that notice. What makes that happen

I don’t know the cause of this issue but I am facing the same problem with replies from exchange.

At least in my environment it looks like the issue is caused by base64 encoded replies. Exchange seems to send base64 encodes responses. For normal SMTP mails and Lotus Notes it works ok.