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Perhaps a simplified dialog will help you. We have two parties
involved… somebody wanting to e-mail you (‘Customer’) and your mail
server (‘mx4creatura85.creatura85.org’, aka 123.45.67.25).
Customer: I need to send an e-mail to creatura85@creatura85.org and to
do so I must find a mail server. Operating System (OS), do you know
where the mail server it?
Customer OS: No, it’s not in my cache or static entries so let me find
out via DNS.
Customer OS: Hey DNS, do you know what in the world creatura85.org is?
Customer DNS: No, let me check around. <time lapse… ten milliseconds>
okay I’ve got it. You want 123.45.67.89. Anything else?
Customer OS: I need an MX record while you’re at it.
Customer DNS: Why didn’t you say so… hold on… <time lapse, five
milliseconds> Okay, for mail (MX) you actually want 123.45.67.25.
Customer OS: Got it. Thank-you. Okay Customer, your message will go
to 123.45.67.25 on TCP port 25. Anything else I can do?
Customer: Great, here is the message. Please give it to ‘creatura85’
at that address.
Customer OS: I’ll be one second… if I’m slow. Time for a trip on the
(Information Super)Highway to deliver the letter. Vroom, vroom.
(“driving” in an ultra-reliable car (probably a Toyota) to 123.45.67.25
port 25).
Customer OS (via TCP): HELO mx4creatura85, I am
customeros.customernetwork.net and I have a letter for creatura85. Do
you know who that is?
mx4creatura85: One step at a time, skippy. Let me verify your identity
first or you don’t get to talk to nobody.
mx4creatura85: Hey DNS… you know this bum who just drove up? His
driver’s license says he’s from 121.212.121.212 but he claims to be from
customernetwork.net.
mx4creatura85 DNS: I don’t know for myself but checking with my
partners at that DNS name they verified the driver’s license is right
for that person.
mx4creatura85: Okay, thanks.
mx4creatura85: All right, Customer OS, you’re apparently who you say
you are. I know ‘creatura85’ and will deliver that for you. It’s been
a bit hectic lately since we opened up for business with the public;
used to keep it all internal and things were much nicer… less
loitering, people going door-to-door asking for random “friends” and
“business partners” of theirs… all kinds of riffraff. Anything else I
can do for you?
Customer OS (via TCP): Nope, that’s it. Thanks.
mx4creatura85: Okay, bye.
Customer OS: Hey Customer, I managed to drop off your letter… took me
about 0.8 seconds. I deserve a raise don’t you think? I didn’t get
confused by any routing issues on either end, and didn’t get snagged by
any network or host-based firewalls preventing access to port 25 on the
remote end. I didn’t even get an old outdated MX record from remote DNS
because of my good reliable contacts. Whew… all in a second’s work.
Customer: Yes, very nice. Get back to work.
mx4creatura85: I’d better store this away so the next time creatura85
comes by I can hand him all of his mail.
creatura85: Hey mail server, have anything for me?
mx4creatura85: Oh boy do I! I just received one from a very important
somebody. Here you go.
creatura85: Great thanks.
So anyway, there are a lot of pieces involved. If your DNS MX entry is
outdated (your responsibility or your ISP’s responsibility to keep up to
date… ultimately yours though) mail won’t get to you because there’s
no way to tell a remote mail server where to send mail. With around
four billion IP addresses available it’s a lot of work narrowing down
the options. Also if you are using a dynamic IP you must make sure that
record updates as your IP does, hence why most sites and services
available to the public online use static IP addresses, or at least have
really well-updated DNS records. You also need to make sure your mail
server is configured to accept data from the outside world, but it needs
to make sure that it does not “relay” messages for just anybody or you
will become a SPAM source overnight. Firewalls need to be out of the
way for port 25 too, of course.
Good luck.
ken yap wrote:
> No, far from that. But you will get more machines trying to connect that
> you might think. You think nobody knows your mail domain, but these
> infected machines will try random addresses.
>
> That’s the other thing you have to get working: you need a MX entry at
> your domain registrar so that the MX record for your domain points to
> your IP address. If your IP address is dynamic, then you need to do this
> at your dynamic IP registrar.
>
>
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