MX record
The DNS record that says which server receives mail for a domain, and the one inbound mail depends on
An MX record (Mail Exchanger) is the DNS record naming the server that accepts mail
for a domain. When something sends to you@yourcompany.com, it looks up the MX record
for yourcompany.com and delivers there. A domain with no MX record cannot receive mail
at all.
Records carry a priority number, lower being preferred, so a domain can list a backup that is used only when the primary is unreachable.
Sending and receiving are separate
Adding an MX record does not affect your ability to send, and verifying a domain for sending does not let you receive. The two directions use different records and different infrastructure, which is why a domain can be perfectly configured for one and not the other.
That separation matters most when you already have mail somewhere. Pointing the MX record for a domain at a new service moves ALL inbound mail for it, so an existing mailbox on that exact domain stops receiving. Use a subdomain when you want both.
In Sendly
Inbound mail is a mailbox feature: you point an MX record at Sendly's inbound host, and messages arriving for that domain become mailboxes you can read over IMAP, over SMTP, or through the API.
The inbound host is Sendly's own, and not the sending infrastructure's — read the receiving email guide for the exact value rather than inferring one.