Failed to send Linkus Welcome Mail - Oversized Packet exceed Router MTU


  • Firmware version: Any
  • Model: S-Series PBX, P-Series PBX

Problem Description

User failed to send Linkus Welcome Mail for Linkus user login even Mail Settings test on PBX is successfully. 




Further analysis show on PCAP dump captured on PBX.  

Try to capture the data by Yeastar PCAP dump during Linkus Welcome Email sending and analyze the package. Filter the data transmission from PBX to mail server with "tcp.port==587" (587 is SMTP port  for some mailbox).

Then you will find much "TCP Retransmission" indication and the MTU length is 1506 which is larger than 1500 set by many routers. Router will discard the data from PBX. As a result, mail server will not able to retransfer the Linkus Welcome Mail from PBX.



On the router side, you might cahnge teh MTU size to a proper value (a greater value in this case). 

On the PBX side, you can customize the MTU packet sent from PBX with following method. However, it requires the Linux knowledge to accomplish. Especially the VI command

If you don't know how to handle this solution. You can try to seek help from your IT or a Linux skilled guy.


1)Login device via ssh how?

2)  Type following command 

vi /ysdisk/support/

3) Press the letter "i" and paste the command to it

ifconfig eth0 mtu 1400 

/**1400 is just an example, you can adjust the value according to your network**/

4) Press "ESC" and enter ":wq" to save the file.


5) Reboot your PBX, and try to send the Linkus welcome email again.



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