To convert PST to MBOX, use a dedicated conversion tool such as SysInfo PST to MBOX Converter. First, download and install the software on your computer. Open the tool and select the PST file you want to convert. The tool will scan the PST file and provide options to choose which emails and folders to convert. Select MBOX as the output format, and start the conversion process. Once completed, the tool will generate an MBOX file that you can then import into email clients that support MBOX, like Mozilla Thunderbird or Apple Mail. Ensure you use reputable software to maintain data integrity during the conversion.