Have the same issue at my employer where I am the I.T. Coordinator. This (421 #CXCNCT) error is not due to a 'blacklist' block, but due to too many concurrent connections being attempted. Cox tells me it is a Siteground configuration issue and Siteground says it is a Cox issue.
Now at first I was thinking it may have been caused by staff sending out 'group' emails even though they were adhering to the <40 recipients in a group rule. However I have tested myself by trying to send a single email from my work accounts ( I have 3) to my home @cox.net account and get the error about 80% of the time. The annoying part is it sometimes takes 2 to 3 days to get the bounce back message from Cox.
Too make things even more interesting, in many cases I (and others) actually receive the emails that Cox's server is saying failed to be delivered.
