[TriLUG] location-specific SMTP problem

[Jonathan Mainguy]

my wife uses AT&T at work and cannot send mail, pretty sure its AT&T
limiting traffic outbound on port 25. If she jumps on the VPN from work,
then all of a sudden email works again. I told her to call AT&T and get it
straightened out, but she has the VPN as a workaround so she has not
bothered.


On Mon, Jul 21, 2014 at 2:49 PM, <matt at noway2.thruhere.net> wrote:

> My initial thought is that the ISP is blocking email traffic except to
> known white listed servers.  Running traceroute, along with NMAP and
> possibly trying to establish some telnet sessions should answer this.
>
> Mentioning the apple products and TLS jumped out at me.  I know from
> experience, e.g. an IPAD Apple doesn't follow the standards for TLS and
> port 25 and instead prefers to use the submission port or SSL.  I had a
> lot of trouble getting the IPAD to work with Postfix - Dovecot based email
> server.
>
> I would also recommend doing some DNS tests, such as running NSLookup and
> trying to retrieve the MX records.  Make sure that this isn't causing
> problems.  Speaking of DNS, make sure you don't have a dud resolution
> occurring (e.g. poisoned cache or simple error).
>
> The fact that it seems to be provider specific really suggests to me that
> something is being filtered.
>
> If you can coordinate have him try to connect and see if anything shows up
> on the server logs at the same time.  Ultimately, this is how I came to
> identify the issue with Apple products and email protocols.
>
>
> > One of my co-workers has problems connecting to our SMTP server from
> > his house. The same Mac laptop works fine at work and most other
> > locations.
> >
> > At home he's on UVerse with an Apple router. If he turns on the hotspot
> > on his Verizon phone, he can connect and send mail. Turn it back off and
> > go through AT&T - fail.  Our office is on TWC BC -- no problems.
> >
> > He can connect to other SMTP servers (GMail) without a problem. He also
> > reports that at certain WiFi spots, he encounters the same problem. He
> > does not know if those spots use AT&T or an Apple router. I've asked him
> > to run a traceroute next time he is on a Wifi spot that fails - to
> > determine if the spot is running through the AT&T network.
> >
> > I connect on port 25 using STARTTLS using Thunderbird, also via AT&T
> > Uverse
> > and have no problems.
> >
> > This seems to narrow down the problem to a combination of our server
> > and either AT&T Uverse or Apple routers.
> >
> > Our server is running dovecot 2.0.9 on Linux 2.6.32-431.17.1.el6.x86_64
> > (RHEL, I think).
> >
> >
> > I've surpassed the extent of my knowledge and it's looking like a
> > long deep hole of troubleshooting ahead  :(
> >
> > Any ideas where I should start looking?
> >
> > I'm thinking I'm going to need to capture the network traffic on his box
> > during an authentication session and dig into the specific messages sent/
> > received. But I don't really know what I'll be looking for.
> >
> > TIA!
> > Chris
> >
> > --
> > ------------------------------------------------------------------------
> -
> > Chris Merrill                           |  Web Performance, Inc.
> > chris at webperformance.com                |  http://webperformance.com
> > 919-433-1762                            |  919-845-7601
> >
> > Web Performance: Website Load Testing Software & Services
> > ------------------------------------------------------------------------
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