auth: Warning: DNS lookup took 1.550 s

Hello,

I am running a dovecot and proxy server on two different virtual machines and on the dovecot proxy server I see around 5-6 times per day the following warning:

Feb 03 16:15:12 auth: Warning: proxy(email@domain.com,xxx.xxx.xxx.xxx,<ABC123456789>): DNS lookup for mailboxserver.domain.com took 1.550 s

I do not really understand how from time to time DNS queries are slow, I tried replicate this issue using dig to resolve the same DNS entry and it was always very fast. Is there any way I can debug better this issue? or is this nothing to worry about really?

Regards ML