First the sender needs to enter the email address of the recipient along with the message using an email application. This should be done at the local computers. Once it is finished and the “Send” button is clicked, the email will be going to the MTA (The Mail Transfer Agent). This communication is done via the SMTP protocol.
The next step is DNS lookup. The system sends a request to find out the corresponding MTA of the recipient. This will be done with the help of the MX record. In the DNS zone, for the receiver address’ domain, there will be an MX record (stands for Mail Exchanger record). This is a DNS resource record which specifies the mail server of a domain. So, after the DNS lookup, a response is given to the requested mail server with the IP address of the recipient’s mail server. This way the ‘to’ mail server is identified.
The next step is transferring the message between the mail servers. The SMTP protocol is used for this communication. Now our message is with the recipient mail server (MTA).
Now, this message is transferred to the Mail Delivery Agent and then it is transferred to the recipient’s local computer. As we have seen earlier, two protocols can be used here. If we use POP3, then the whole email will be downloaded to the local computer and the copy at the server gets deleted. If the protocol used is IMAP, then the email message is stored in the mail server itself, but the user can easily manipulate the emails on the mail server as in the local computer. This is the difference when using both the protocols and this is how your email gets delivered. If some error occurred to send the email, the emails will be delayed. There is a mail queue in every mail server. These mails will be pending in the mail queue. The mail server will keep trying to resend the email. Once the email sending fails permanently, the mail server may send a bounce back email message to the sender’s email address.
This explains why you maybe getting bounce back emails sometimes. The reason for bouncing back will be explained in the message. There are many reasons for getting an email to bounce back such as as incorrect email address in the ‘to’ field.
https://www.interserver.net/tips/kb/exactly-emails-works-steps-explanation/
What is SMTP?
SMTP is part of the application layer of the TCP/IP protocol. Using a process called "store and forward," SMTP moves your email on and across networks. It works closely with something called the Mail Transfer Agent (MTA) to send your communication to the right computer and email inbox.
SMTP spells out and directs how your email moves from your computer's MTA to an MTA on another computer, and even several computers. Using that "store and forward" feature mentioned before, the message can move in steps from your computer to its destination.
https://whatismyipaddress.com/smtp
250 – This SMTP server response simply means everything went well and your message was delivered to the recipient server.
450 – Your message was not delivered because the other user mailbox was not available. This can happen if the mailbox is locked or is not routable
https://sendgrid.com/blog/smtp-server-response-codes-explained/
Mail servers can be expressed in DNS servers with the MX records. MX stands for Mail Exchanger. MX records specified the Domain name related mail server IP address. This mail server accepts mail with smtp protocol from senders.
To retrieve domain MX recordssimply use MX option in combination to domain name you wish to query.
Use the +short option to retrieve only mail exchange (MX) records:
If you need to troubleshoot your own DNS server locally while the domain's name server is not yet set, you can point dig to any local or remote DNS server you wish to query by using @HOST/IP syntax.
https://linuxconfig.org/how-to-check-domain-s-mx-mail-exchange-records-using-dig-command-on-linux
Email Client
SMTP Server - mail carrier
Hello Command
MTA - local post office
DNS - to map out path
SMTP Server
Email Client
user agent(UA) - email application such as outlook, thunderbird
web-based email - gmail etc
private email system - thunderbird outlook etc
UA - MTA1 - MTA2 - UA
MTA uses SMTP to communicate each other
MIME Header - what the email contains, txt image etc
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