Is it advisable to allow pop3 through internet

My network environment does not have pop3 server nor Exchange pop3 email
Basically, no network or Microsoft require POP3 service.
Recently, some of the user requested to for pop3 service as they claimed
that they cannot access thier internet using email client via pop3. They do
not want to used web-based as it is not so user as email client.
What are the risk if i allow pop3 access to internet?