Mail Server reject incoming mails

[NickDungee]

hello,

i configured a mailserver and i have a reject issue....some mail adresses come in and some not.
i created mail accounts and include this accounts in apple mail. i can send or receive mail's, but not from every adress.
i get the following response:

Reason: Remote SMTP server has rejected address
Diagnostic code: smtp;554 5.7.1 <[email protected]>: Relay access denied
Remote system: dns;mx00.udag.de (TCP|17.158.236.237|49636|62.146.106.39|25)

i have a root server and a extern domain (united-domains). i created a a-name / mx entry but the dns (mx00.udag.de) ist not my server....its the united-domain dns. what is the problem?

sry for my bad english!!!

thank for help, best regards