# Understanding Internet mail.

## dE_logics

I've been referring to articles to understand Internet mail, and they don't appear to form a consensus with things that happen practically and the things that Wikipedia says (which appears to form a consensus with how things work in the real world).

All articles will ignore mail submission agent. Saying the email client (or mail user agent) directly connects to the mail transfer agent (also called a mail server) which has an MX record.

However in my Thunderbird client, which I use with my gmail account, the outgoing server is set to smtp.googlemail.com which has no MX records, suggesting that it's a mail submission agent, which non-wiki source ignore.

I've come to the conclusion that a transfer agent and submission agent is basically the same thing; the difference being that the submission agent does not have an MX record.

I think in this system (client -> MTA/MSA -> MTA ), there are 3 servers involved, 1st the submission agent, then the transfer agent of the account from which I'm sending mails from (in my case, it's my Gmail account); this server can be found with gmail.com MX records. This transfer agent forwards the mail to the transfer agent associated to the server which handles the destination email address (for e.g. ymail.com); it finds out about the server from the domain's (ymail.com) MX record.

Is this how it works?

----------

## papahuhn

 *dE_logics wrote:*   

> I've come to the conclusion that a transfer agent and submission agent is basically the same thing; the difference being that the submission agent does not have an MX record.

 

You can't generally say that. Google and other big providers may have a dedicated MSA Address for load-balancing and high availability reasons, but it is not unusual to have MTA and MSA handled by the same server software instance. Postfix, for example, can listen on port 25 and on submission at the same time, where the submission port is configured differently - namely with authentication, mailfilter-bypass and a "may-relay-to-different-domains"-policy.

Edit: Meant MSA but wrote MDA.

----------

## dE_logics

 *papahuhn wrote:*   

>  *dE_logics wrote:*   I've come to the conclusion that a transfer agent and submission agent is basically the same thing; the difference being that the submission agent does not have an MX record. 
> 
> You can't generally say that. Google and other big providers may have a dedicated MDA Address for load-balancing and high availability reasons, but it is not unusual to have MTA and MDA handled by the same server software instance. Postfix, for example, can listen on port 25 and on submission at the same time, where the submission port is configured differently - namely with authentication, mailfilter-bypass and a "may-relay-to-different-domains"-policy.

 

For simplicity, lets remove the delivery part, and focus on the sending only.

I'm saying that Mail transfer and submission agent are the same thing.

----------

## papahuhn

 *dE_logics wrote:*   

> For simplicity, lets remove the delivery part, and focus on the sending only.

 

I thought we did that already.

Edit: I see, I wrote MDA where I meant MSA.

 *Quote:*   

> I'm saying that Mail transfer and submission agent are the same thing.

 

Same thing by what measure? Postfix can be both at the same time, on different TCP ports and different configuration. Wikipedia states that "there are also programs that are specially designed as MSAs without full MTA functionality". From Thunderbird's point of view, an MSA is anything that requires authentication. A well configured MTA will only accept emails destined to its own well-known domains (or from trusted sources, i.e. MSAs), because there is no authentication on 25/tcp.

----------

## dE_logics

 *papahuhn wrote:*   

>  *dE_logics wrote:*   For simplicity, lets remove the delivery part, and focus on the sending only. 
> 
> I thought we did that already.
> 
>  *Quote:*   I'm saying that Mail transfer and submission agent are the same thing. 
> ...

 

Ok, so the difference between the 2 is that MSA has features by which the MUA can authenticate itself with it; were as MTA strictly forwards mails to the destination server.

Is this okay?

----------

## papahuhn

 *dE_logics wrote:*   

> Ok, so the difference between the 2 is that MSA has features by which the MUA can authenticate itself with it; were as MTA strictly forwards mails to the destination server. Is this okay?

 

"can authenticate" -> "must authenticate", but yes, that's about it. You can read RFC 6409 for further details, it is not that long.

----------

## dE_logics

 *papahuhn wrote:*   

>  *dE_logics wrote:*   Ok, so the difference between the 2 is that MSA has features by which the MUA can authenticate itself with it; were as MTA strictly forwards mails to the destination server. Is this okay? 
> 
> "can authenticate" -> "must authenticate", but yes, that's about it. You can read RFC 6409 for further details, it is not that long.

 

Yeah, now it clears things up.

Thanks!

----------

