This issue occurs because the MTU size of the router/switch/server of the data to be transferred is different from the MTU size of IMSVA.
The packet is dropped when it is too large for the MTU path. Thus, the sending host retransmits the packet by fragmenting it to smaller packets. If there are too many dropped packets while retransmitting, the connection will fail and the message will not be sent.
When sending a message fails on your first attempt, MTA retries to send the message at a later time. However, the path for resending may change due to various processes (e.g. load balance, congestion, outages, etc.). At some point, the IMSVA server may find a path with enough MTU to successfully send the message to the destination server.
To resolve the issue, decrease the MTU size on the network interface of IMSVA:
- Log in to IMSVA via SSH.
- Go to /etc/sysconfig/network-scripts.
- Determine which interface is used by IMSVA by running the following command:
- Identify which eth# uses the IP address for outgoing connection.
By default, it is set to eth0 (e.g. vi ifcfg-eth0).
- Edit the interface ifcfg-eth# and add the parameter "MTU=1472".
- Save the file.
- Restart the network interface using the command below:
service network restart
- Verify if the issue is resolved. If not, set the MTU value to a lower one (e.g. 1450, 1425, 1400, etc.) until the issue is fixed.