Could be the messaging side of their Outlook program. Their IT would need to take a look at the Windows program and any updates to it recently.
I am having same issue was working fine and now all of a sudden can no longer send vtext messages. I also noticed that now Verizon has a new integrated messaging service and I am wondering if this killed Vtext.
I'm having a similar problem. I e-mail four messages per day to my phone (iPhone 6, iOS 8.3) from an automated mailer running on justhost.com. Nothing has changed in the system since March, and I was receiving messages each day until May 1--none since. I can send messages to (my number)@vtext.com from other services (e.g., me.com), and I can send messages to (my number)@vzwpix.com from justhost.com. If messages from the same source to my number @vzwpix.com get through, but none to @vtext.com get through, it appears that Verizon is blocking the e-mail. I checked my phone's text block settings, and there are none blocked, and e-mail and web blocking are off.