You CAN insert one or more images directly from Photos into a Mail message using an Outlook account.
Create a new blank email message in Apple Mail using your linked Outlook account.
Enter the appropriate information in To: and Subject: fields
Move the cursor into the large text area of the message.
Tap the large < symbol in the lower right corner of the screen above the keyboard.
You may also see 'autocorrect' and suggestions for any text already entered into the body of the message before the < symbol
On the next screen, tap the Photos image (second from the left, one photo overlaid on a blank rectangle):
Photos on your iPhone will open and allow you to select one or more images from the thumbnail images of those pics saved in Photos on your phone.
When you tap on a thumbnail image, a blue checkmark will appear on that image:
Click the 'X' in the upper right corner of Photos when any or all images are selected, to close Photos and return to your Mail message screen.
When you're finished composing the message and click the Send icon, you will see a choice of image sizes to send, smallest first, largest (full original image size) last. If you're sending several images, that feature can help you avoid sending attachments larger than the recipient's email provider will accept.
