To get to this page, go to Settings → Diagnostics → Log File Sizes
This page lets you set the maximum log file sizes in one place for the various log files which VPOP3 makes.
The file sizes will not be exact.The technical details below may help if you are interested.
When VPOP3 creates a log file, it creates an empty file of the appropriate size and keeps track of how far through the file it has written. This is to try to avoid excessive disk fragmentation which can happen if several files are appended to in an interleaved manner. When VPOP3 creates, say, a 10MB file, Windows will try to place it in a single contiguous block on the disk, which won't happen if the file starts off with zero length and then grows gradually. This means that if you view a log file directly, it will probably have lots of empty space at the end because that part of the file hasn't been written to yet.
When VPOP3 has written data to a log file which takes it over the maximum size for that log file, VPOP3 will rename the current file to <something>.LBK and create a new <something>.LOG file of the appropriate size. If there was already an LBK file, that will be deleted (unless Retention rules say otherwise). This means that you will always have the current log file (which may contain between zero and <max> bytes of log) and the previous log file which will contain at least <max> bytes.
If you want to store more logs, then the Retention tab lets you set log file retention rules. In that case, you can tell VPOP3 to keep log files for a certain amount of time, or a certain number of log files. In this case, rather than deleting old LBK log files, VPOP3 will rename the LBK file being replaced to <something>_<date>_<number>.LBK. The <date> will be the date when the LBK file was renamed and the <number> will be a sequential number for the log files on that date. You can use the last-modified timestamp on the files to see what times' log entries are recorded in which files.
The retention rules will also tell VPOP3 how much disk space can be used by these log files and how much free space there must be left after creating the log files. In this case, VPOP3 will delete the oldest retained LBK file to meet the disk space requirements. (It will not delete the <something>.LBK log file, just any extra files it has kept).