emails sent to comcast accounts being rejected

As the President of a small non-profit organization, I use a Microsoft supplied email account that ends in .onmicrosoft.com for company business.  Emails sent from this account are being rejected with the following reply:

resimta-a2p-650768.sys.comcast.net rejected your message to the following email addresses:

xxxxxxxx@comcast.net
Your message was rejected by the recipient email server. Please check the recipient's email address and try resending your message or contact the recipient directly. For more tips to help resolve this issue, see DSN code 5.1.0 in Exchange Online - Office 365. If the problem continues, contact your email admin.

resimta-a2p-650768.sys.comcast.net gave this error:
<xxx@obinh.onmicrosoft.com> sender rejected

This has happened with messages sent to one of my Directors, one of my volunteers and even my personal Comcast email address.