If lotus is just an SMTP server (i'm not totally familiar with it), you can just set the Lotus SMTP settings up for your Email. Kentico takes an SMTP address, username and password and uses it to send emails out.
Also programatically you can send emails "from" anyone, so if you just want the emails to show they came from their email account that's in Lotus, you can just set the From address to their email.
If the above doesn't help you and i'm totally off the mark, and Lotus now has some form of API to send emails through, you can use a Custom Loader Module and catch the global for emails and perform the code needed. I don't necessarily see a specific email event to hook into, so you may need to use the generic Object.Insert event and detect email object updates.