Error 530 means your email client isn’t authenticating correctly when sending email. This usually happens if your email client isn’t set up for SMTP authentication.
SMTP authentication ensures only authorised users can send email through our servers.
To fix this, enable SMTP authentication in your email client settings. If you’re not sure how, see your email software’s help section.
SMTP settings
Outgoing Mail Server: mail.btinternet.com
Port: 465 (this may not be automatically populated on selecting SSL, so you'll need to check)
SSL Encryption: Enabled (but not STARTTLS)
Authentication: PLAIN
Username: your email address, including the @btinternet or @btopenworld.com part
Password: your btinternet or btopenworld password
For details of email settings, see: BT Mail: What are the settings for outgoing and incoming mail servers?