The distinctions between POP and IMAP get blurred with advanced settings. POP was designed to remove email from the server but you can opt to override that purpose. IMAP was designed to sync the server with multiple clients but you can opt to override that purpose.
OP is confused about the protocols because the following statement doesn't make sense: "I have outlook set up as POP so that it will not be removed when I use webmail to delete it." If you first download via POP, there's nothing to remove from the server...unless you have a "POP override" enabled in advanced setting to keep it on the server.
OP just needs to review the advanced setting in Outlook.