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 mai...
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