Outlook now has a new feature to compress ALL attached images in one go. Use Insert – Attachments option to add images. BEFORE sending the mail, open the File menu. You will see the new option. Choose the Resize large images when I send this mail option. Now all images will be compressed by reducing their size to 1024X768 or smaller. Aspect ratio will not be disturbed.
DO NOT use this option if you want to retain original resolution. If the mail bounces because the other person(s) cannot receive such large emails, put the files on some cloud storage (OneDrive, Google Drive, Dropbox, etc.) and send a link.
This feature was released in May 2017. Depending upon the configuration decided by your Office 365 Administrator, you may receive this feature much later. You will get it instantly if you are on the Office Insider – Fast branch.