Question
Can my resolvers cause slow DNS lookup queries?
Answer
Absolutely! Your server's resolvers are the set of nameservers to which your system will perform DNS queries to resolve IPv4 & IPv6 addresses.
This means that if you have a resolver which is experiencing issues, or has a poor connection between your server, you may experience issues with DNS resolutions such as timeouts or delays.
If this delay is long enough, it can cause Exim to extend the lookup long enough to where Webmail applications may appear to not work due to the long resolution time.
To address this, you will want to modify your resolvers:
/etc/resolv.conf
You can simply use your ISP's provided DNS resolvers, or you may opt to use public DNS resolvers such as Google, 8.8.8.8, 8.8.4.4 or Cloudflare 1.1.1.1, 1.0.0.1.
Special note & warnings:
Usage of public resolvers such as Google or Cloudflare may incur rate-limiting from the provider depending on their usage policies.
Additionally, RBL's such as Spamhaus will not work properly when using a public DNS resolver.