Email is complicated but the jist is there are relays and mailbox/mail-exchanger servers (and email clients). And a million different anti-spam measures that make making sure people actually get you email difficult.
When you send an email you mail client contacts you mail-exchanger (MX) and drops the mail in your outbox. Then the MX will look up the MX record for the domains in the TO field and attempt to send the email to it using SMTP.
The first thing receiving server ussally does and look up the IP of the sending server and see if it's on a spam black list. If it is it will probably just drop the connection. Then it will look up the SPF record for the domain in the FROM address and see if the sending server is allowed to send that mail.
Larger email services will have an internal 'reputation' scoring system that will use data from reported spam to figure out what IP's and domains are sending spam emails and filter them out. They'll also look at if it's a residential IP, in an IP pool for a major cloud provider etc. Each provider has a different system and it can be really difficult to get a provider to trust your IP or whitelist your IP so you can make sure that your mail actually gets to who you're trying to send it to.
Relays are pretty simple they'll take email from one place and send it to the recipient. The mail exchanger usually has a built-in relay. A lot of people will use a third party relay service so they don't have to worry about managing the reputation of the IP of their sending mail server. They'll just add an SPF record for the relays service to their domain. And then configure their mail exchanger to send all outbound mail to the relay and then the relay will do the MX lookup.
A lot of mail services will also have their MX records pointed at relays. These inbound relays will often have a lot of those anti-spam services bolted onto them and they can also be used for load balancing to make sure that the service is always able to accept mail even if it doesn't make it in the mailbox immediately.
When you send an email you mail client contacts you mail-exchanger (MX) and drops the mail in your outbox. Then the MX will look up the MX record for the domains in the TO field and attempt to send the email to it using SMTP.
The first thing receiving server ussally does and look up the IP of the sending server and see if it's on a spam black list. If it is it will probably just drop the connection. Then it will look up the SPF record for the domain in the FROM address and see if the sending server is allowed to send that mail.
Larger email services will have an internal 'reputation' scoring system that will use data from reported spam to figure out what IP's and domains are sending spam emails and filter them out. They'll also look at if it's a residential IP, in an IP pool for a major cloud provider etc. Each provider has a different system and it can be really difficult to get a provider to trust your IP or whitelist your IP so you can make sure that your mail actually gets to who you're trying to send it to.
Relays are pretty simple they'll take email from one place and send it to the recipient. The mail exchanger usually has a built-in relay. A lot of people will use a third party relay service so they don't have to worry about managing the reputation of the IP of their sending mail server. They'll just add an SPF record for the relays service to their domain. And then configure their mail exchanger to send all outbound mail to the relay and then the relay will do the MX lookup.
A lot of mail services will also have their MX records pointed at relays. These inbound relays will often have a lot of those anti-spam services bolted onto them and they can also be used for load balancing to make sure that the service is always able to accept mail even if it doesn't make it in the mailbox immediately.