Email Sending Servers are defined by a domain in DNS as an SPF record and as part of a domain ANTI FORGERY policy
The SPF record may specify 1 or billions/trillions of IP numbers that may relay email as the domain.
Or, some domains simply say that EVERYONE may relay email as that domain.
Usually security aware institutions, like for example: commercial banks (in first world countries), clearly and specifically define the exact IP numbers that may relay email as that bank.