Traditionally, Internet mail servers accepted mails destined for another network, a process known as "relaying". This was in keeping with the co-operative, open nature of the Internet and meant that anyone could use any mail server to send mail, although using a local server is generally more efficient. Relaying is useful to many groups of users, unfortunately, one of those groups is spammers. They can use the relaying on another ISP's server to send their junk all over the World. For this reason, many ISPs have disabled relaying on their servers, which means those servers will only accept mail address to or sent from their network.
A growing number of ISPs are refusing to accept mail from any server that is an open relay, in the hope that it will encourage all ISPs to disable mail relaying. However, as well as its unpleasant use for spamming, mail relaying has several genuine uses. U-NET have to leave relaying enabled on send.mail.u-net.com so that users of their Global roaming service, who will be dialled in via another ISP, can still use the server without having to alter their configurations. This makes it a target for mail server "blacklists".
The other mail server, mail.u-net-com is not an open relay and is not found on these blacklists, but can only be used when dialled into your U-NET/Wirenet account. Unless you also want to use the server when connected from elsewhere, you should change your configuration to use mail.u-net.com for all mail operations.