I'm running Fedora 26 with the latest updates (Dolphin 17.04.1). I use Evolution as my default email client but I don't think it's related. If I right click any file in Dolphin and select "send as email attachment" nothing happens. If I run Dolphin from the terminal I get org.kde.pim.kidentitymanagement: IdentityManager: There was no default identity. Marking first one as default. stringToMailbox: Invalid address org.kde.pim.templateparser: html string is empty for extracting to plainText org.kde.pim.templateparser: Impossible to extract plaintext
As far as I know, that service item is added by KMail, not Dolphin, so expects that KMail is setup correctly.
You are right. I just removed Kmail completely and now the service menu is gone. Sorry about that.