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Changing MTU setting fixes problems sending email

Changing MTU setting fixes problems sending email

CNET staff
2 min read
Alan Remos writes: "Ever since updating to Mac OS X 10.1.5, I have had problems sending out emails through Verizon (who now requires SMTP authentication when sending out mail). I eventually found out that Verizon is using an MTU (Maximum Transmission Unit) setting of 1492 instead of the standard MTU 1500. You can set the MTU manually in Terminal by typing: <sudo ifconfig en1 mtu 1492> but this change is not preserved after a restart. A permanent solution is to open (as root; you can use Pseudo for this) the file located at /System/Library/StartupItems/Network/Network and add the following line to the "Configure interfaces" section: <ifconfig en1 mtu 1492> (for AirPort) or <ifconfig en0 mtu 1492> (for Ethernet). Then restart. After I did this, the Verizon problem no longer occurred."

Frank Sauer and Michael Ford had previously alerted us to problems sending email with attachments via a Dual-Ethernet AirPort Base Station setup. This too turned out to be related to MTU settings, as noted in several Apple Discussion Boards threads (see one, two, and three, for example).

See also our previous coverage on MTU settings and routers - which notes that this problem may be restricted to PPPoE conections.

Update: A reader alerts us to this page, which also covers the MTU issue and provides a similar solution, using AppleScript.

John Strung adds: "I posted a MacFixIt Forums thread with detailed instructions on how to use the OS 9 version of IPNetTuner to set the MTU to 1492. For people using OS X who don't want to get involve in Unix commands, IPNetTunerX from may be a solution. Finally, for people using routers, the easiest solution might be to set the MTU in the router instead [Ed. note: as we discuss in the link cited above]."