Very simply, you tried to send email to a user that doesn't exist (well, the user may exist, but the account doesn't). Verify the recipient's information and try again, or ask the recipient if they have another address you can send to.
Articles in this section
- Email Rate limits
- Setup a new email account in MacMail or Apple Mail
- Email account setup in Microsoft Outlook 2013 (SMTP/IMAP)
- Reducing My Email Disk Space Usage
- iPhone SSL won't turn off
- How to forward email in Mozilla Thunderbird?
- How to reply to email in Mozilla Thunderbird?
- How to delete email message in Mozilla Thunderbird?
- How to send email using Mozilla Thunderbird?
- How to check for new email in Mozilla Thunderbird?