[ALUG] Alternatives to SMTP when the network fails

Mark Rogers mark at quarella.co.uk
Wed Dec 8 14:47:48 GMT 2010

Can't think of a good subject for this, sorry!

I have a customer with an ADSL line and direct delivery of email via SMTP to 
their external IP (into Exchange). The MX record for the domain points to one 
of my servers running Postfix, which basically forwards mail to their IP if it 
is up.

So far so good....

However, they've just suffered a BT-inflicted ADSL outage that's lasted 
several days and they want to have a backup for the future. They're going down 
the 3G backup route (using a router which falls over from ADSL to 3G), but 
this will give them a dynamic IP when on 3G and, more importantly, we've found 
out the hard way that in general 3G providers do not allow us to make incoming 
connections (whether on port 25 or any other).

So, my question is: what is the best solution to this?

I have come up with several options:
- Move the email to POP3 delivery, ie mail ends up in a POP3 mailbox on my 
server, they collect periodically from that box. From experience this causes 
problems with Exchange, although I don't recall what exactly.
- Provide some kind of POP3/IMAP access to the Postfix mailqueue, so in normal 
use mail comes via SMTP, but when they switch to 3G they can access the 
mailqueue (using POP3 or IMAP, and then by extension via webmail) as a 
temporary measure, with the mail coming in via SMTP as normal once the 
connection is restored. However, I have no idea if this is even possible?
- Drop everything to a POP3 mailbox on the server, but configure fetchmail to 
deliver it to them via SMTP if the connection is up. Sort of half-way between 
the two above options.
- Setup a VPN between the mail server and the exchange server, and deliver to 
the VPN address. This seems like the "best" option although I have no idea 
whether it is practical having never tried it. I'd assume OpenVPN as the best 
VPN solution? I also don't know how well it would scale if I want to then do 
this with lots of other sites?

Mark Rogers // More Solutions Ltd (Peterborough Office) // 0844 251 1450
Registered in England (0456 0902) @ 13 Clarke Rd, Milton Keynes, MK1 1LG

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