Mailbox Provider Checker

The MX record tells you who decides whether your mail arrives

Every domain publishes, in public DNS, the server that receives its mail. That record identifies the provider, and the provider is the thing that will judge your message. Treating a Google inbox and a Microsoft inbox as the same target ignores the single biggest variable in whether cold email lands, and the information costs nothing to collect.

Two providers, two different filters

Google leans on engagement signals: whether people open, reply, and mark you as wanted. Microsoft leans harder on sender reputation and content heuristics, and is generally less forgiving of a new sending domain. The practical result is that the same campaign at the same volume performs differently across the two, and lower per-mailbox volume is the usual answer on the Microsoft side.

Gateways are a separate answer

When the MX points at Proofpoint, Mimecast or Barracuda, there is a security product screening mail before any mailbox sees it. Those domains are not impossible, but they convert far worse, and knowing which rows they are stops you drawing conclusions from a campaign that was filtered before delivery.

No MX means guaranteed bounce

A domain with no mail server cannot receive email at all. Those rows are pure bounce risk sitting in your list, and this is the cheapest way to find them, since it needs no mailbox check and no account.