I do have the same issue her with NS 7.3.1611, SOGo Version 3.2.7 in combination with Lightning (Thunderbird 52.2.1 (32-Bit) and Lightning 188.8.131.52) on caldav calendars only!
* on local calendars I can send invitations mails
* Sogo Web-Interface also doing it correct
* caldav calenders can not send email invitations at all (greyed out option)
Searching the web this problem seems to be from SOGo, maybe
SOGoAppointmentSendEMailNotifications setting but so far I could not get any solution on that.
Can you reproduce this? Any idea what's wrong?