The recipient just sees a normal email sent from you. Yesware doesn't change where your email is sent from so there are no "sent on behalf of" messages that can trigger spam filters.
If you have Tracking turned on, your recipient might see (depending on their email client) a message such as "Load images from this user?". This message is triggered by some email services because Yesware tracks using a 1x1 pixel image to track email opens. This is the same pixel-beacon approach that all major email newsletter/marketing companies use to track emails.
We find that recipients usually have their email preferences set to display images by default so likely would not see this message.