Yesterday we noted a strange phenomenon suddenly appearing in the past few weeks where sending messages above a certain size (with attachments or not) through Outlook Express is problematic.
We also noted that MTU problems cause severe lags in network logon times, cause email attachments and other functions within Outlook to fail, and cause applications to stop functioning entirely. However, we only provided a solution for switching the MTU setting in Mac OS X, allowing only those running Outlook Express in Classic mode to apply the workaround.
For those of you looking to apply the same settings under Mac OS 9, check out IPNetTuner, which can modify a variety of network paramaters including MTU.
MacFixIt reader Ross explains another scenario where modifying MTU settings can resolved issues:
"Having jumped into the OpenSource camp I now have a Debian Woody (2.2.20) gateway serving my LAN at home via ipchains. Once I got it setup all seemed well however after a while, sometimes ages - sometime straight away, connectivity would seem to be lost. ppp0 was still up and I could ping and DNS was alive however nothing 'flowed' back to my local machines in the way of actual data. It was very confusing.
"A bit of digging (tcpdump to the rescue) and I figured I needed to alter the MTU on my 7600 which was sending tcp packets at 1500. IPNetTuner to the rescue - all works great now with an MTU of 1452. Interestingly enough my wife's Win98J laptop seems to be auto negotiating to 1452 - no registry digs were needed."