I recently replaced a faulty Telkom ADSL modem (Marconi) with a new one (Billion 7300) at one my clients...
They use Alt-N's MDaemon (http://www.altn.com/) as their mail server on a Windows Server 2003 machine.
Mail is delivered directly using Dynamic DNS and MX records, something like:
@ 3600 IN MX 10 clientname.dyndns.org.
The entire setup was working perfectly (for over a year), until I installed the new ADSL modem...
Since I installed the Billion 7300 ADSL modem, my client was complaining that they were not receiving e-mail from certain people, and that some e-mail that they were sending was not being delivered.
After much investigation and hair-pulling, I eventually noticed that the MTU setting on the new ADSL modem was set at 1472.
As soon as I changed the MTU setting to 1492, everything was working flawlessly again!! And my client is happy again.
Now, I have a few questions for those of you that know about this stuff:
1) What exactly is the MTU, and why is it so important?
2) Why was my client only able to receive some of their e-mail, whilst the other incoming mail was bouncing back to the senders as undeliverable?
3) Why was my client's outgoing mail only being delivered to some of the recipients?
4) Is the MTU value of 1492 optimal for a SAIX ADSL connection, or should I set it to something else?
In a nutshell, I need someone to explain to me why setting the MTU at 1492 works, and why 1472 did not work!!
Thanks in advance,