This exception can be thrown only if DNS caching is enabled (Enabled is true). In this case, MailBee keeps track of all DNS MX lookup requests made and the following attempts to connect to SMTP MX servers designated by the DNS records retrieved during DNS MX lookup. If, for certain domain, all DNS MX records failed (all SMTP MX servers did not respond), MailBee remembers in its DNS cache that this domain is dead and marks the corresponding DNS records in the cache as disabled. All subsequent attempts to connect to this domain will immediately throw this exception without wasting time on waiting for a response from the dead server. The same optimization is performed when non-existent domain is encountered (i.e. the DNS server returned an error).

However, when Timeout interval (by default, 90 minutes since the moment of adding a record into the cache) elapses, MailBee removes such DNS MX records from the cache. It's assumed the failed server may be back during this time, or DNS records may change, etc, so it's worth trying to check the availability of the host in question again.

Note
This exception does not refer to the error which occurred just now. It rather occurs to due to past knowledge (acquired during previous send attempts and saved in the cache) that SMTP MX server (which was determined via DNS MX query) could not be contacted or the domain does not exist.

Reference