This error usually occurs when there are typos in the recipient's email address. Some common errors include the following:
- Quotation marks: <'username@gmail.com'> or <"username@gmail.com">
- Dots at the end of the address: <username@gmail.com.>
- Spaces before or after an address: < username@gmail.com>, <username@gmail.com >
If you're sure there are no mistakes, the Gmail address may have been disabled, deleted, or never existed in the first place.