The SMTP (Simple Mail Transfer Protocol) allows the sending of emails on the Internet. The mail delivery process does not differ much from the classic mail system, since there is an "organization" that is in charge of sending the mail following a series of steps so that it finally reaches the recipient. Within this process, the SMTP server is a computer that acts as an "electronic postman", which is responsible for making deliveries to those who correspond.
When you send an email from your mailbox (called MUA - Message User Agent) to a specific contact, the message is generally dispatched to an SMTP server using Port 25 (for example smtp.servidor.com), the which is configured in your mail client and which acts as MTA (Message Transfer Agent or Message Transfer Agent).
The client and the server are then connected, and the server verifies all the information regarding the transmission of the message (sender, recipients, domains, etc.). If the domain to which the recipient belongs has their account directly connected to the server, the mail is delivered immediately. Otherwise, the SMTP server delivers the message to another server where the recipient is located.
Now what happens if the recipient's server is down or busy? The SMTP server delivers the message to a backup server (if one exists) and, if none is available, the email is queued for output and then retry delivery. After a certain period, if the delivery is not successful, the message returns as "Not sent". Otherwise, the message will reach the recipient's inbox.
The SMTP mail servers associated with providers such as Hotmail, Gmail and Yahoo are shared among many users. This means that you could share an IP address with a spammer, which can affect the deliverability of your emails. In addition, providers limit the number of emails that can be sent, either per hour or per day.
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