Email send blocked

I have several email accounts configured on my iPad including Apple iCloud. When I am connected via my t-mobile in home wifi network and I try to send a message from iPad to the Apple iCloud account it gets a "mail delivery failed" message with reference to proofpoint.com web site.

However the message does get sent to other devices (that are not using T-Mobile WiFi).

If I change the wifi on my iPad to a different network, then the send works also.

Why would this happen, and how can I fix it?