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[suse-security] Mailing Trouble ?!



Hi !

I run a mail server at a university institute. Recently I noticed that mails from and to a certain site disappeared without notification. After mailing to the sysadmin of that site he told me that his mail server software had given up sending mail to my server after having received an error code 421 whichhe says means that the server is temporarily unable to accept SMTP connections. His system seems to be either unwilling or unable to switch to the alternate mail server in my MX records. 
My problem is, I don´t know if there is anything wrong with my system. As far as I know, this is the only case where my domain had trouble receiving or sending mail. We experience sluggish mail traffic at times here, but according to my university´s computing center that is their problem because their server are running near full capacity.
As far as I have been able to find out, my DNS service is configured correctly (Name resolution and reverse resolution work, my MX records are correct and can be accessed from outside) and my domain is not listed on a blacklist.
My questions : 
1) Is the inability to establish mail communication to and from this specific site my problem or theirs ?
2) How can I check if my system yields this error-code 421 message ? If so, what can I do to correct it ?
.
My server is a Linux box running SuSE 8.0, sendmail version is 8.12.2.

Thanks for the help,

Jörg