[MM3-users] 421 - Too many sessions from source IP

My new MM3 server is receiving these errors from cox.net:

"(delivery temporarily suspended: host cxr.mx.a.cloudfilter.net[34.212.80.54] refused to talk to me: 421 cxr-ibgw-6004a.stratus.cloudmark.com cmsmtp too many sessions from 213.125.4.246. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXCNCT) "

The link gives just this:

421 - [too many sessions from <source IP>] The sending IP address has exceeded the five maximum concurrent connection limit. Configure your sending server to establish no more than five concurrent connections.

1. Is that a legitimate excuse to refuse passing on emails or is cox.net too stingy?
2. Is that a setting we need to change in Mailman somewhere? Or should this be handled in some Postfix setting?

Yours,

Allan