Iridium has advised that this issue has now been resolved.
Posted Apr 09, 2018 - 19:59 UTC
Identified
We have been advised by Iridium that there is currently an issue affecting Short Burst Data (SBD) mobile terminated (MT) messaging. Iridium are working to resolve the problem as quickly as possible.
If your device relies upon the Iridium bearer, messages sent from the Portal to devices may be affected.