I have the same issue on a 200 cabinet. Have to reset the main cabinet to get it working again. Last time happened was in November, and just got call today that emails stopped working. Tried to get into remotely and cannot but customer was able to log into it locally via ssh so will go onsite tomorrow. Changed external smtp hosts from yahoo business to gmail back in November and thought it was fixed, but still happening.

Have you tried changing the mobile messaging from the asc card to the sip card?