Here are the step by step guide on -
First of all, open the mail app on your iPhone.
Click on the compose new email key. (In the bottom right corner)
Type the email of the person to whom you want to send the email, messages, and attachments.
Click on the Up Arrow(At the top right corner).
Now you'll get a popup telling you that the attachments may be too large and send them with a mail drop. Click the "Use Mail Drop" option to send the email.
For more read -