Many organizations use single sign-on (SSO) for security purposes as well as helping their employees more easily access the software applications they use most frequently. These credentials, however, can cause issues when Yesware attempts to authenticate with your email address since the credentials are different. This may send you in a "login loop" whenever you attempt to activate Yesware. The same issue happens if your company uses a VPN tunnel to connect to a Wi-Fi network.
A simple workaround is to disconnect yourself from your SSO network/VPN tunnel and reconnect to a different network or Wi-Fi hotspot and then go through the activation process again. Once that is done, you will be able to use Yesware.