I run a small home network with two older Linux machines and an XP laptop.

Recently I ran a Windows network analysis tool and after testing various 
URLs it reported my best MTU to be 1500, as this is the default for ADSL 
I thought nothing of it and set my router to 1500 (It had been 1472).

Everything appeared to be fine, web surfing OK, receiving mail OK but 
had a problem sending emails with attachments; also there was one web 
site that I couldn't access on my windows or Linux machines.

As the only thing I recalled altering was the MTU changed it back to 
1472 and now all fine.

I know the default is 1500 also for the Linux machines so is this a 
common problem with incorrect MTUs that most will work OK but some areas 
of the network may not.

Geoff Lane