Now I'm becoming a pest!

I get periodic 'temporary lookup failures'. Does anyone know what could cause that? It doesn't happen all the time. Aren't the lookups happening on the mail host machine - like the local LDAP?

snippet from messgage:

In: MAIL FROM:<> SIZE=16244640
Out: 250 Ok
In: RCPT TO:<>
Out: 451 <>: Temporary lookup failure
Out: 221 Bye