Slow delivery of verify your email - only with optusnet domain

Hi there,
First week in of using Auth0 as our MFA software and our users are experiencing issues receiving their verification code email within the 5 min timespan we have set, this is ONLY happening to our users with optusnet.com.au domain …
They do receive the email, but not within 5 mins so the code expires, they click resend and the cycle begins again.
I cannot see anything in the topics re optus … we don’t really want to up our timeout setting, is there anything we can do from an admin side?
Any help most welcomed.

Clare