[exim] Usage of dnsdb primary mx

Is it possible to retrieve the highest prioritized MX record using a
dnsdb lookup? If so, how?
How would dnsdb affect a server's load? We're talking about a fallback
platform for approximately 500+ domains - and growing.

Are results of a dnsdb lookup cached? If that may be the case, for how
long are they cached?

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Thomas Klaver