Emails not leaving Outbox on iPhone (Optus 4G)

[#1]

Hi All,

At my work place we are currently unable to send HTML (Signatures etc.) emails on Apple Devices connected to the Optus 4G Network. Plain text emails are not affected. When connecting the phones Wifi - the emails send fine.

The error message we get is 

"Cannot Send Mail. The message was rejected by the server"

Our email's are hosted via Microsoft Exchange. 

We have tested with multiple iPhones and an iPad and receive the same error. 

To me it sounds like some sort of Outgoing Mail Server issue or Optus network is getting blocked somewhere. 

Any assistance or ideas would be very much appriciated. Thanks!

[#2]

Optus wouldn't be blocking you from sending HTML based emails considering all emails from iPhone are HTML based by default.

Given you are using Exchange to send/receive emails, the fact it isn't going through, are you on the work WiFi or using 4G/3G mobile data? I would be referring to your IT department to check their end, as exchange won't be touching the Optus mail systems.