Have you ever wondered what happens to your emails after you press send?
Your email takes a journey to the mailbox of the recipients, via various queues and services; it may even travel through the Internet to reach its destination.
Initially your message is put in your Outbox waiting to be sent. Almost immediately it is picked by the Mail Submission service and its journey begins. This service notifies another service called the Store Driver about your message. The Store Driver does 2 things; it puts a copy of your message in your Sent Items and transfers your message to the Submission queue.
Once your message reaches the top of the queue it is processed by the Categorizer. This service sorts mail messages and decides where to send them. To do this, it looks at the recipients and contacts the Active Directory to find out which server holds the recipients mailbox. Having done that it works out the best route for the message, it may simply be a case of delivering to the mail server or it will identify the next place (often referred to as next hop) to forward the message onto. The Categorizer will then put the message in the relevant queue. If your message is destined for several different places, it will create multiple copies and put one copy in each queue. The contents of each queue will be sent out using the SMTP (Simple Mail Transport Protocol).
Whilst being sorted your company’s email policy will be applied to your message, which could include checking it does not violate any of the rules such as exceeding the maximum size of attachment or being addressed to blacklisted recipients.
If your message includes recipients who work in a different organisation, your email will normally have a disclaimer added after your signature and it will put into the Internet queue. Each Internet bound email, will need further routing information which is acquired by contacting a DNS (Domain Name Service) server, to obtain the IP address of the recipients mail server. The contents of the Internet queue will be sent to your company’s firewall or router device to be sent on via the Internet.
The majority of emails are sent internally and these emails will be delivered directly or routed to the recipients. The Active Directory service will have provided the Categorizer with the name of the mail server holding the recipients mailbox. The message will be put into the relevant queue for delivery to that server or an intermediate server if there is no direct route. The DNS server will again be used to get the IP address of the server or next hop. There is also a queue for local delivery, which holds emails to be delivered to local mailboxes.
When your email reaches its destination, it is put in the queue for local delivery. Mail items in the local delivery queue are placed in the recipients Inbox by the Store Driver service. Your email has reached its destination.
This description is of Microsoft Exchange Server.
Judy Marshall is a Microsoft Certified Information Technology Professional (MCITP) in Enterprise Messaging, who is currently training graduates entering the IT industry at FDM.
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