Query times and recursive-clients

[Alex]

Hi,

I have v9.4.2 running on Linux and I'm seeing a bunch of messages in
my mail logs like the following:

 reject: RCPT from unknown[xxx.217.8.156]

Trying to later resolve this IP returns a valid hostname, so I'm
concerned that there is perhaps a timeout value that is too low for my
system, which may be overloaded at times, or some other limit is being
reached that may impact results.

What can I use to determine if this is even actually a problem?

I don't see anything peculiar in the logs, and memstats looks pretty
uneventful. Is there some other stats that I should poll to determine
if an upper-bound may be being hit?

Thanks,
Alex