The MailUtl3 receives your mail client's command, transfers it to the actual POP3 server, receives its response and transfers it to your client. The MailUtl3 relays commands and responses. The MailUtl3 adds several information in the mail header.
Since most of e-mails transmitted from end user's subscriber line, not relayed ISP's mail server, are spams, the MailUtl3 is one of spam filter program.
The MailUtl3 is not a content filtering program.
It does not check the context of e-mails.
However, you must maintain the symmetry function in your mail program.
It would be a white list.
Since MailUtl3 runs DNS queries, it increases the network traffice in the Internet and loads of DNS servers.
It takes time while MailUtl3 is running DNS queries.
You do not mention it if you receive e-mails from your mail server at fixed interval such as every ten minutes.
However, when you returned to your home from your office, you would be anxious the waiting time caused by DNS queries.
It would be absolute end when the amount of e-mails in the mail server exeeds three hundred.