Emails with attachments on mobile devices not sending - stuck in outbox

[ANDREW LEESON]

As of the last few days, emails that are sent via mobile devices that HAVE attachments will not send. They just sit in the OUTBOX. It doesn't matter what size attachment it is. 1k, 3MB, etc.
Outlook clients are fine on desktops and laptops, no issues. (mix of Outlook 2016 & 2010)
Running Exchange 2010 
All the certs are fine on the hub transport servers, nothing has been changed nor expired.

No recent server updates

restarted the servers, IIS.
If you send an email with an android or apple with no attachment it's fine.

I am out of ideas here...
Thanks

[#2]

Is this happening to all users?

What mail app is used on the smartphones?

Are they WiFi connected or simply on cellular signal?

Does regular mail send out normally?

Have you tried running a trace on the message?

Does this happen to internal e-mails  and/or external e-mails as well?

Let us know,

Michael