Hi Ken,
Thank you for posting your problem, I am here to help you today. Possible reason you are getting this problem is cause of incorrect outgoing (SMTP) server settings or a typo in the sender's email address
Try this step to help fix this problem: Go to settings on your iPhone and Under "Passwords & Accounts" look for the email address you are using where you got the error. Click on the account and if the options are grayed out, the email's password must be reentered. This will prompt you to reenter your password via an extra option present under the grayed out options. The username is the email address, and then type in the password for the email address. Once completed hit “Done” and try sending another email.
Let me know how this goes. Looking forward to helping you out. If you need more help please feel free to let me know.
Best regards,
Jansher