If you receive the following error when sending: There are non-ASCII characters in the local part of the recipient address, you are probably using accented characters in the email address. The server you are trying to send through cannot handle non-ASCII characters.
Additionally, you could also have this message stuck in your Outbox folder, whereby you'll receive this message on each message send operation. To check for this, open the On My Mac account (on macOS) or Local Folders account (on Windows), then navigate to the Outbox. Look for any offending messages stuck in the Outbox and fix the recipient addresses for this message.