The IMAP protocol isn't intended for sending emails, but on some servers, you can move a message to the sent folder and this causes it to be sent. This is how the Send Test Email function (New Message) works in the IMAP test.
If you use Send Test Email (New Message) with an IMAP server that doesn't support this functionality, then the sent folder will fill up messages from every test run, so we advise only enabling this option if your server supports it.
If you need to test sending emails and your server doesn't support sending with IMAP, we suggest using the SMTP Test type instead.