All e-mail validation levels but RegexCheck utilize DNS MX lookup and thus need some DNS servers (at least one) to operate.
MailBee can auto-detect which DNS servers are available in the system. The developer can use Autodetect method to retrieve DNS settings from OS or from the config file.
If multiple DNS servers specified, MailBee will distribute load between the available DNS servers to improve performance. However, if some servers have lower Priority than others, they will not be tried unless primary servers fail.