Emails and servers

How is it that some of our clients are having difficulty sending and receiving email to our other clients who are on different servers, within our 'group' But external emails ( such as home ISP's) can get thru with no difficulty.

IE

If I send an email FROM my account on server ABC to a client who is on server DEF, it doesnt get thru. The reverse is also true. BUT. If I send an email from my ISP account, it gets through no problem. This is true on all of our servers. I recall hearing that it has something to do with a NS/DNS configuration, but I cannot for the life of me recall what it is /was.

 

 

 

 

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