On 29/09/15 21:47, coolhandluke wrote:
On 2015-09-29 05:13, Robert Sharp wrote:
I have been trying to use a third-party backup provider to cover
occasional outages (SiXwishlist). I set up an MX record as instructed
but I get no emails sent on when my server recovers. I asked them to
check what was happening and they told me my port was not open. It
seems they were expecting postfix to be listening to port 4000. I
asked for clarification and got a response that they would send emails
on 587, 2525, 3000, and 4000, but that 25 was not recommended and "is
blocked for 90% of service hosting providers".
as wietse mentioned, it sounds like they are used to acting as the
primary mx for customers whose providers block 25/tcp inbound. in
those cases, they receive mail on 25/tcp on your behalf and then
connect to your mail server on an alternate port (such as one of the
above) that is not blocked by your provider.
this can -- and does -- happen often enough that there is a market for
this service.
I thought that smtp/25 was the backbone of email systems. I use 587
internally for submission so I don't see why I would make it
accessible to the outside world. Do I need to open another port? Have
I got port 25 wrong all this time? Or do I need to get myself a proper
backup provider?
you're correct and you're doing things the right way, afaict. 25/tcp
*is* the standard port for mta to mta delivery and 587/tcp *is* the
standard port for mua to mta delivery.
probably the easiest thing to do in your particular case is to make
your mta available on one of these non-standard ports that they will
attempt to deliver to -- in addition to, not instead of, 25/tcp.
there are a few different ways to accomplish this but the simplest, in
my opinion, would be to simply forward incoming connections to
2525/tcp from their mail servers to 25/tcp on your mail server.
if you are using linux & iptables on your mail server, it's as easy as
adding rules such as the following (substituting in the ip
addresses/ranges of their outgoing mail servers, of course):
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 2525 -s
x.x.x.x/yy -j REDIRECT --to-port 25
hth,
/chl
Thanks for the advice. I asked the provider about this and they
eventually told me that had enabled a port 25 backup server (just for me
it seems). I adjusted my MX record and Virgin kindly arranged an outage
to test the whole thing. Works like a dream!
