Email activity status is still showing "Sending" in CRM 2016,even though user already received the email.

Hi,

Email activity status is still showing "Sending" in CRM 2016,even though user already received the email.

And user started receiving the same email more then once, until we delete this activity in CRM manually.

" To Email id" or address used ,is not a contact or even not linked address to any profile in CRM.