Newbie question on sending mail via telnet

[Melpa]

Hi

I have a query that I am trying to figure out that I hoped someone on
this forum maybe able to help me with.

Sometimes, if we can't send email to a certain domain, we'll try
sending a message via telnet to see what response we get back.

So, let's say the remote domain is Domain.com, and their MX record is:

mail.domain.com 10
mail2.domain.com 10
backup.domain.com 20

Let's also say that mail.domain.com has all its possible SMTP
connections open. From my understanding of MX, the following should
happen:

i) My mail server tries to send mail to mail.domain.com, however this
server replies back with a "4xx" too busy response

ii) Mail mail server should then immediately try mail2.domain.com

iii) If mail2.domain.com is too busy, then my mail server should try
backup.domain.com

However, how does this all work if I am trying to send a mail via
telnet? If I attempt to send a mail, using telnet, to mail.domain.com,
at what point would I get back the "4xx" too busy? At the moment I
type "EHLO me.mydomain.com" or when I actually send the message?

And, if sending via telnet, would it only try the initial server I am
telnetting to, or will it automatically fail over to mail2.domain.com
[sorry - I know this sounds like a silly question but I have no way of
testing]

Any help appreciated!

[Sanford Whiteman]

> i) My mail server tries to send mail to mail.domain.com, however this
> server replies back with a "4xx" too busy response
>
> ii) Mail mail server should then immediately try mail2.domain.com

Actually, you should assume that MXs of the same priority will be
tried in random order (there are exceptions not worth getting into
here).

> However, how does this all work if I am trying to send a mail via
> telnet? If I attempt to send a mail, using telnet, to
> mail.domain.com, at what point would I get back the "4xx" too busy?
> At the moment I type "EHLO me.mydomain.com" or when I actually send
> the message?

It depends! If the server knows it will be busy for all messages, you
will get it even before you type EHLO. If it is too busy for messages
from your domain, you'll get it after your MAIL FROM. It some
mailboxes are too busy, you'll get it on RCPT TO.

> And, if sending via telnet, would it only try the initial server I am
> telnetting to, or will it automatically fail over to mail2.domain.com
> [sorry - I know this sounds like a silly question but I have no way of
> testing]

You have no way of testing? I don't understand. You certainly have:
telnet <host> 25.

Telnet sessions are to a single destination host. You are the
SMTP-sender when you use telnet, so you are responsible for following
the RFC. You don't choose to try another MX, your manual "session" didn't
follow RFC.

-- Sandy