Who knows SMTP?

RWB

OK so one of the number one annoying things I come across at the help desk I work with is SMTP trouble. Mind you, the help desk is for Hotel's networking equipment, we're not an ISP; instead we take their internet connection and put in the equipment necessary to allow the guests to use it via Ethernet, DSL or Wireless(DSL or Ethernet).

SMTP is pretty strict, so guests who travel a lot I would assume needs to change their SMTP information so they can send email from place to place unless they use web mail. Apparently, we use a relay server which IIRC handles this on it's own so that people don't normally have to change their SMTP server; for the most part at least.

Naturally, from time to time, we get a guest who needs to input the SMTP manually which is normally internally routed via the server on site. I'd say this works half the time, the other half the time I get to hear them whine about how "it works everywhere else". Luckily I had yesterday off and this same lady kept calling in and said that her friends made a bet with her that we couldn't help her. Her friends won becuase we already did all we could for her the day before(when I took the call).

Now from what I do know is that a relay server can become black listed? Which some servers may use to prevent spam, but oddly it can work one moment, then not the next, then an hour later it may start working again, I just don't know. Or something to do with reverse-DNS lookup and the server refuses the connection isn't really coming from who it claims to be.

This is just an utter nightmare and I want to understand it, so any insight would be great. If I left any holes of info please note it and I'll try to explain better. But when I get a guest who says they can't send email, my heart just sinks. I wan to help these folks, but what I want even more is them not yelling at me. I am so sick of hearing "it works everywhere else" or "then why was it working an hour ago!?"

drasnor

Just stuff I've run across as a user:

1) In my experience, even if you run your own SMTP relay server, I can't use it unless I manually put the address in, except with regard to clever network wizardry on your part.

2) There are a whole lot of SMTP relay servers that require their clients to be on their corporate network etc. in order to process their mail. Typically this means VPN but for the less computer literate it means trouble.

3) There are even more SMTP relay servers out there that don't care where their clients are. I use one, it's nice. Too bad my campus filters all SMTP traffic that isn't directed to their SMTP relay server.

4) A goodly number of the SMTP relay servers I've used require secure authentication via SSL. The port used for this kind of SMTP relay server is different than the default port and if there's any lingering configuration for one when you go to a open SMTP relay server it usually messes up.

5) When my SMTP relay server is blacklisted I usually get a message back from the receiving server stating that the message was rejected. I used to get this a lot right after my mail provider was bought by Tucows but I imagine AOL users get it a lot more.

6) Thunderbird may act like it can register multiple outgoing mail servers but it really can't. It always uses the top one in the list without regard to whatever you set your default server to.

-drasnor

mtrox

RWB wrote:

Now from what I do know is that a relay server can become black listed? Which some servers may use to prevent spam, but oddly it can work one moment, then not the next, then an hour later it may start working again, I just don't know. Or something to do with reverse-DNS lookup and the server refuses the connection isn't really coming from who it claims to be.

I may have that one RWB. I used to use Comcast as my smtp. Due to their size, and maybe their incompetence, they get blacklisted a lot. I had the same thing, sometimes I'd get blocked, other times not...even when sending to the same ISP. I found that blacklisting is per IP address. And if you do an nslookup on smtp.comcast.net, you get several IP's (see attachment). I would guess that is true of other large smtp servers. So when you send to the URL of the smtp, it all depends on the luck of the draw as to whether you get one of the blacklisted IP's.

Ryder

RWB,

Also check with the ISP for the hotel and ask them not to block outgoing port 25 requests.

Many ISP's block port 25, except the SMTP servers that are on their network (meaning you have their internet service, so you can use their SMTP)

When you are not at home...the network you are on may not allow that connection because they block port 25.
See if the ISP can help since you have the equipment there to manage traffic anyway.

Kwitko

On top of all that, you've got greylisting and SPF (Sender Policy Framework).

mtrox

RyderOCZ wrote:

Many ISP's block port 25, except the SMTP servers that are on their network (meaning you have their internet service, so you can use their SMTP)

So far since I moved my email hosting to GoDaddy 2 years ago, I have never been blocked anywhere, and my laptop gets around. They smtp out of either port 80 or 3535. I think that's part of the trick.

RWB

Well there are multiple ISP's these hotels use, naturally we can do nothing about what their real ISP is doing if they are blocking port 25... but they are definitely not supposed to be and some of these hotels seem to work fine for most people. Of course if it works for them then they don't call in so I don't know for sure :P But of a hundred people using an internet connection at one place alone I am sure more than 1 or 2 are trying to use their email, but those 1 or 2 people are the ones that call in.

Of course this is also taking in the number of people who just don't want to deal with it even if they are having trouble.

Also, most of what you guys have mentioned so far is a bit above me, I can kinda understand it by context and because I have seen it before, but I've not had to mess with it so I don't really know or understand it.

troll

EMail... Much Fun...

Local ISP's dont really matter...

You put a centralized stmp server at your NOC or in one of the hotels, call it "smtp.hotel.com" or whatever domain is available for use... Has to be a FQDN so it can be found on the internet. The SMTP server has to be on an ISP that will allow you to send SMTP traffic.

Allow only the IP's from the participating hotels to send through it. (relay control in the SMTP setup)

When customers arrive at any hotel in the chain all they have to do is set their SMTP to mail.hotel.com and send away...

The mail will leave the Boston hotel, connect to the Detroit SMTP Server and away it goes to where ever as the Boston hotels IP list is a valid realy for the Detroit server. (For Example)

Just a quick off the top of my head, if there are issues for this then I need more details about the setups...

We host 35+ (small potatoes) email / web / dns domains at work and it can be a real nightmare... sometimes...

RWB

We do have an internal SMTP server here in the office that some sites go through, and it's the same problem. Works for us of course, but not the guests.

troll

It's all relay control, if you add the IP's of all the networks that are allowed to send mail through your server then the problem goes away...