Email Receiving Servers are defined in DNS as the MX records for a domain.
The MX records tell other email servers where to deliver email for a domain.
Incoming email for any domain will not function without an MX record.
Outgoing email for any domain is managed with SPF record. Also see Email Sending Servers
Usually, system and network administrators closely check their incoming and outgoing email server reputations, as low reputation scores may result in incoming and/or outgoing emails not being delivered to main inbox but to a spam folder or even discarded.