connection_limit_reached

[Edward]

I have a client who has an Exchange server setup at his premises.
Mail getting to his server comes up with this error at times.

in /var/qmail/control/ smtproutes  i have      domainname:[64.110.110.166]
in /var/qmail/control/rcpthosts i have domainname

delivery 56991: deferral:
Connected_to_64.110.110.166_but_greeting_failed./Remote_host_said:_421_uga1n
t01.domainname_connection_limit_reached/


what could be the cause of this error in the logs..


Edward