Assuming that your users are local to your Exchange server, they won't directly notice anything. If you send an email in Outlook, it'll go into their Sent Items folder almost instantly. Where it will have an impact is on the Exchange server delivering the emails to their destinations. Depending on what sort of volumes you experience, the uplink may be an issue if your users are sending more email than the uplink can deliver timeously. This will cause your queue to grow, leading to delays in email delivery.
In addition, you must bear in mind that a saturated uplink will also impact your downlink. Even if you are mostly downloading data, IP packets are being sent constantly, especially when you're dealing with TCP applications such as HTTP, because the client doing the downloading is also sending packets to the server all the time to acknowledge receipt of each TCP packet.
So essentially if you saturate your uplink it's possible that your connection can become unbearably slow before you even get close to saturating the 6Mbps downlink.