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It is important to understand the entire mail flow from sender to receiver. It helps to understand the entire mechanics and workings of all the several components that exists in the mail flow topology. The understanding of this will help you to troubleshoot why some email cannot be send or why you cannot receive certain emails.

A - User compose a  email using Mail Client. The email is sent from mail client to mail server via SMTP, MAPI or ActiveSync. The Mailbox maintain constant connectivity
B - Mail Server check for outgoing mail security rules/policies
C - If YES, then check for Internal Mailbox
D - Deliver to Mailboxes
E - If uncached or new MX Domain record, then query MX from DNS Server
F - Sender Mail to Receiver Mail Server via SMTP (25, 2525) or SMTPS (465) or SMTP/TLS (587)
G - Received Mail Server check SPF records
H - Mail Server check for incoming mail security rules/policies
I - Mail Server check for valid user mailbox
J - Mail Server deliver to User mailbox
K - Mail Client maintain constant connectivity between Mail Client and User Mailbox
L - Mail Client check for incoming mail security rules/policies

SMTP (Simple Mail Transfer Protocol) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today. Mail servers and other message transfer agents use SMTP to send and receive mail messages. Proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as Outlook.com, Gmail and Yahoo! Mail may use non-standard protocols internally, but all use SMTP when sending to or receiving email from outside their own systems. SMTP servers commonly use the Transmission Control Protocol on port number 25.

SMTPS (Simple Mail Transfer Protocol Secure) is a method for securing the SMTP using transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality.
SMTPS is not a proprietary protocol and not an extension of SMTP. It is a way to secure SMTP at the transport layer, by wrapping SMTP inside TLS. Conceptually, it is similar to how HTTPS wraps HTTP inside TLS.




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