Why am I getting 'sender address was invalid' error when sending email?

Usually because the sender address is invalid.

Might also be some add-on app, such as a ‘coffee shop” VPN, or add-on security app, getting tangled.

Some more info, if unfamiliar: If you can’t send email on your iPhone or iPad - Apple Support

Jol58 wrote:

Cannot send mail the sender address was invalid

[Re-Titled by Moderator]

Maybe you are trying to send the email through an email server using an email address from another email provider. e.g. you are trying to send an email through a gmail server using an outlook account email address as sender.