How do servers send and receive email?

[Feathers McGraw]

That's so true! Nice to be able to mess around with something that isn't too important, too.

I just bought another one (No.3), with case and charger I paid £43. Not sure how much they are in the US, but it won't be a lot!

[Robtygart]

No I haven't even priced one.

Well the command should tell you what port is open. Best way to learn is to scew around.

[Feathers McGraw]

Thats really cool I would love to do that!

Yeah they're amazing little computers. Do you own one? I found Citadel really easy to configure, couldn't get postfix + dovecot + squirrelmail to work completely.

Thanks for the command but the reason I'm asking is to learn, the server already works perfectly!

[Robtygart]

The mail server is running on a raspberry Pi; I'm happy to leave it running 24/7/365 because it costs me practically nothing.

Thats really cool I would love to do that!

If you have your server setup, try "nmap" using

nmap -v -A youraddress.com

or use the IP address.

It should show you the list of ports.

[Feathers McGraw]

Just a guess, but one would think you use the port you set your server up with, probably a 'typical' port, here's another guess, you will have to leave it running 24/7/365 or you are going to get some bounced emails, I hope you have a reliable ISP that's ok with the constant connection and bandwidth, they might consider an email server as 'business' and slap some nasty new fees on you. I've thought about doing what you are doing, might turn this machine in to a server when I build my new one.

Please read what I asked again. Obviously, the software is going to use the port you tell it to - I'm asking whether that port is important or arbitrary, and why.

The mail server is running on a raspberry Pi; I'm happy to leave it running 24/7/365 because it costs me practically nothing.

Yep, my ISP is reliable. Since I've been with PlusNet, I haven't ever noticed my connection drop out. A few emails a day is hardly "constant bandwidth" of the type that would make an ISP uneasy.

They couldn't possibly consider it to be "business" use because I don't own a business or run one from my home, and I use my work's email via webmail for work related emails. Again, the volume of traffic is not very high. I doubt it has even registered with them.

Try it yourself, it's quite liberating.

Feathers

[tek_heretik]

I'm trying to get my head around what happens when you send & receive emails from an email server at home.

I think some of my confusion is around port numbers, and what happens if you change the defaults.

Say someone wants to send an email to foo@yourdomain.com - as I understand it, their email client will look up an MX record for yourdomain.com from your DNS, which will tell them the right IP address to send the email to. They then make a connection... but on which port? If you change the port in your server's settings, how do the emails get through?

I believe people with ISPs that block port 25 can still receive mail by changing port nos, is this because the email is sent to the ISP's server and then transferred the last little bit over a different port, allowing ISPs to monitor all email on their system?

Similar story for outgoing mail - if you use a non-standard port to send, do you need to route the mail through another server like your ISP's mail server that will send it on from there on the standard port?

Feathers

Just a guess, but one would think you use the port you set your server up with, probably a 'typical' port, here's another guess, you will have to leave it running 24/7/365 or you are going to get some bounced emails, I hope you have a reliable ISP that's ok with the constant connection and bandwidth, they might consider an email server as 'business' and slap some nasty new fees on you. I've thought about doing what you are doing, might turn this machine in to a server when I build my new one.

[Feathers McGraw]

Thanks, I've read that before, but the example given on Wikipedia isn't exactly the same as my usage case.

I'm asking what happens if I use my own email server, a step that isn't included in the Wiki example. Does my server connect to "smtp.a.org" (or similar) as the first link in a chain, or does it replace it?

If it replaces it, is this why it's a problem when ISPs block port 25, because then your server is unable to do step 4? If the message was sent using SMTP on a different port, would it get delivered or not? Does it depend on whether the server at the other end is listening for incoming mail on that port?

Feathers

[Snowhog]

An explanation: http://en.wikipedia.org/wiki/Email#Operation_overview

[Feathers McGraw]

I'm trying to get my head around what happens when you send & receive emails from an email server at home.

I think some of my confusion is around port numbers, and what happens if you change the defaults.

Say someone wants to send an email to foo@yourdomain.com - as I understand it, their email client will look up an MX record for yourdomain.com from your DNS, which will tell them the right IP address to send the email to. They then make a connection... but on which port? If you change the port in your server's settings, how do the emails get through?

I believe people with ISPs that block port 25 can still receive mail by changing port nos, is this because the email is sent to the ISP's server and then transferred the last little bit over a different port, allowing ISPs to monitor all email on their system?

Similar story for outgoing mail - if you use a non-standard port to send, do you need to route the mail through another server like your ISP's mail server that will send it on from there on the standard port?

Feathers