CXN must have an SMTP server to relay the mail to, you cannot do this purely with POP3.
If the users have CoS permissions (as well as network access to reach the appropriate port) they can use IMAP4 to connect their mail client directly to CXN in addition to their normal mail solution. Be advised that this can impact system capacity at scale, esspecially if the mail client doesn't support IMAP IDLE.
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