This issue of email not sending in MS Outlook 365 or Outlook 2016 or other Outlook renditions can be brought about by numerous elements, some of them are: 

Enormous size connections can stop the email fix can't send mailing measure. 
MS Outlook account secret key was changed as of late 
The record was not validated with the mail worker 
Outlook worker or mail worker remains disconnected 
There is no default E-mail account present 
Send setting of the record are tainted 
The PST or OST Data are gotten to by another program 

The active program is at present checking the active messages.

Read more here:- https://www.datarecovo.com/unable-to-send-emails-from-ms-outlook/