I found a bug in smeserver-sogo wich is maybe not relevant to the contribs, because I saw the same behavior with another product.
let me explain
sogo is able to send invitation by email to attendees when you use the webmail, but not when you use a client like thunderbird+lightning, I saw the same with K9.
In fact when the 'email from' field is not customized (Sogo Setting/Mail/Imap settings/email setting) then it is workable, invitation made by lightning in thunderbird are sent to attendees, but if I set a customized email from, then sogo stops to send email invitation to events.
Sure that if your first domain (set in the console) is the good one, and not a dummy one, then you have no problem, but if you need for certain users to set a different email domain in the sogo settings ('email from' field), then you will have some troubles.
Any clues accepted