Hi Jens,
We don't have a problem, both exist happily together. Our external sales people use twiggi or webmail while on the road, also they can use Outlook to send/receive. In their accounts in Outlook we have checked the option 'My server requires authentication' for outgoing mail server, then 'use same settings as my incoming mail server' - This will depend on how you set up your secure mail, there are several options!
Hope this helps,
Martin.