You are trying to send email from a mac.com email address using the legacy pipex smtp mail server.
That's never been approved by TalkTalk but you would have been successful in the past in doing so by using unauthenticated mail sending via port 25. But port 25 is now blocked to prevent the sending of unauthenticated mail. That also blocks mail sending from non-TalkTalk Mail domains.
Please note that all TalkTalk mail domains including dsl.pipex.com email addresses must use the smtp.talktalk.net outgoing mail server using port 587 and tick to use TLS/SSL for the STARTTLS command to start the connection for authenticated TalkTalk domain email that's TLS 1.2 encrypted. TalkTalk also requires outgoing SMTP authentication by User Name (full TalkTalk email address) and TalkTalk email Password. Full details are in TalkTalk's Email Support Hub. Note that your iMac email software must support TLS1.2 encryption which means the OS must be version 10.12 Sierra or newer*.
TalkTalk does not authenticate the sending of non-TalkTalk mail but you can set up an email account on a *supported Mac PC to use the outgoing secure mail servers designated for the relevant email domain. For example, here are the secure email settings to use for mac.com email addresses:
Incoming Server - IMAP
Server: imap.mail.me.com
Port: 993
Security: Use TLS/SSL
User Name: Your full mac.com email address
Authentication Password: Your mac.com email password
Outgoing Server - SMTP
Server: smtp.mail.me.com
Port: 587
Security: Use TLS/SSL
User Name: Your full mac.com email address
Authentication Password: Your mac.com email password
Let me know if you need any additional help.
