How smtp work?

[Yong Shiuh Rong]

just wonder, if I want to send a file (1 MB) to 10 person, when it reach the mail server, how large is the file?

how really smtp server work??

[Niels Endres]

Your email client will send a 1.3MB file to the mail server. A number of things can happen there, but the usual practice is that 10 copies are made, and each person receives their own copy.

[Stuart Teo]

Your smtp client, normally an email program like Outlook Express or Mozilla Thunderbird, will contact the smtp server that is listening at port 25. (unless the smtp server administrator changed that default configuration) We will not discuss smtp over SSL here.

Your smtp client then sends the email to the smtp server. The smtp server then contacts the DNS server to ask for where it should send that email. (MX record)

It then sends it to the "destination" which is also an smtp server. We won't go into the details of MTA and etc here.

Email program packages email generally the same these days and attachment are send out MIME. (Multipurpose Internet Mail Extensions)

If the 10 people you're sending the email to are on different domain, e.g. one is someone@hotmail.com and the other is someone@yahoo.com, then 10 copies will be sent out. (Because each MX is different) In this situation, there will be 10 copies of the same mail in the smtp's spool directory.

If the 10 people are at the same destination MX, different smtp servers will behave differently. It could be 1 copy or 10 copies.

[Yong Shiuh Rong]

thanks Niels and Hwee Liang, your explanation is loud and clear.