Does MTU matter?

Recently, I have changed my ISP to HKBN. I chose the plan BB10 which provide 10Mb upload and download speed. This is much better than ADSL. For such high bandwidth, I want to use my NSLU2 Linux box to setup a web server at my home.

However, there is some problems. I found that not every machine can access my web server. Ellen using Pacific Supernet cannot access the web server. Besides, a friend of mine using Netvigator faces the same problem. However, the W3C's HTML validate can access my web server to validate my HTML files. Moreover, my company can also access the server.

This is really strange. Today, I try to tackle the problem. I tried to set the server in DMZ at my router but fail. Then, I look for other setting in the router's configuration. I find the item "MTU". MTU stands for "maximum transmission unit". Currently setting is 1500 which is the maximum allowed value. I try to change the value smaller to 1400. It works! However, I want to ask, "Does MTU matter?".