What an MX record does
MX (Mail Exchanger) records tell the world which servers accept email for your domain. When someone replies to your cold email, their mail server looks up your domain's MX records, picks the host with the lowest priority number and delivers the message there.
If MX records are missing, replies bounce and you lose the very conversations you were emailing to start. If they point to the wrong host, mail lands in a mailbox nobody reads. For cold email, working MX records on every sending domain are non-negotiable.
MX records for Google Workspace and Microsoft 365
Google Workspace now uses a single record: smtp.google.com with priority 1. Older setups still show five aspmx.l.google.com hosts; both work. Microsoft 365 uses one record shaped like yourdomain-com.mail.protection.outlook.com with priority 0.
Google Workspace
1 smtp.google.com
Microsoft 365
0 yourdomain-com.mail.protection.outlook.comInboxOne publishes these automatically when it provisions a mailbox, along with SPF, DKIM and DMARC, so a domain is reply-ready the moment it is added.
Why MX matters for deliverability, not just receiving
Receiving mail servers check that the domain in your From address can actually receive mail. A domain with no MX (or an MX that does not resolve) looks disposable, which is exactly what spammers use. Gmail and Microsoft both treat it as a negative signal.
- Every sending domain should have valid MX records, even secondary domains used only for outreach.
- The MX host must resolve to an IP and answer on port 25.
- Avoid pointing MX at an IP literal; use a hostname.
- Keep MX on a reputable provider. Shared-hosting mail servers are frequently blacklisted.
