How to investigate "Unable to resolve hostname" error at bundler in Officemail

Bundler stopped processing jobs.
Error messages in the Bundler and SFTP log:
Unable to resolve hostname

Customer's  internal DNS server stopped resolving addresses for the bundler thus preventing bundler from updating Admin service from any new activity that happened on the server.

Once the DNS server address resolution is restored system should start normal operation.
Environment Details
Product Feature: General

Operating System: Not stated

Database: Not stated

Configuration: Not stated
UPDATED:  April 13, 2017