kumomta/docs/tutorial/checking_logs.md

Check for success in the logs

Regardless of whether the mail delivers or not, you should take a look at the logs. The easiest way to see what is going on iw with the built-in tailer utility. Simply open a new terminal (so you can work on other things and watch logs at the same time) and run this:

sudo /opt/kumomta/sbin/tailer --tail /var/log/kumomta

This assumes a default installation with the logs located in /var/log/kumomta/.

If you want to dive in deeper, compressed logs are found in /var/log/kumomta/ as can be seen in this tree. Logs are segmented by a combination of size/time and stored in compressed files named after the time that the segment was started. To read these, you need to unpack them first. You have many options for configuring logging.

/var/log/kumomta
├── 20230311-033705
├── 20230311-033844
├── 20230312-182542
└── 20230314-181435

We can take a look at a specific log by decompressing it and since these are zstd compressed, you can view all but the current one with zstdcat. ZSTD is a streaming compression utility so the current file cannot be accessed until it is flushed and closed. You can force the current log to close by stopping KumoMTA.

Using the example above, we can see the content of the newest file after stopping KumoMTA with a zstdcat /var/log/kumomta/20230314-181435.

The default for log segments is to flush them after 1GB of data is written, but you can adjust them to flush after a certain amount of time. kumo.configure_local_logs has all of the available logging configuration options.