ekeventeditcontroller is not sending invite mails to invitees for the event created randomly

Hi i am creating events in my calendar using the native ekeventeditcontroller. However when i am adding invitees, for some emails a question mark is shown on the side and when the event creation is done, the invite mails are not going. This is randomly observed. I tried an exchange email and gmail and to both emails the invite email is not going. Is this something that we are able to controller if yes how. Attached is a screen of the ekeventeditcontroller.How to debug this issue as i beleive this is done by the ios iCalendar natively.