Unable to send email to Comcast email addresses

We are a small non-profit with about 100 volunteers. Administrators use Microsoft 365 email to send information to volunteer's personal email accounts using a MS 365 Distribution List. Email sent to volunteers with Comcast email addresses are not getting through. The error message is:

Delivery has failed to these recipients or groups:

John Doe (xxxxxx@comcast.net)
Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.

Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.

Diagnostic information for administrators:

Generating server: BY3PR18MB4676.namprd18.prod.outlook.com
Total retry attempts: 5

xxxxxx@comcast.net
Remote server returned '550 5.4.300 Message expired -> 451 4.2.0 Throttled - https://postmaster.comcast.net/smtp-error-codes.php#RL000010'

NOTE: Email sent to an individual Comcast address (i.e. not using the distribution list) does go through.