on 06-10-2018 10:40 AM
on 06-10-2018 11:04 AM
The error message and the circumstances point to the TalkTalk spam protection blocking your sending. Normally you can send to up to 50 recipients every hour. But that's using fully authenticated secure email. And if there's any recipient email addresses that are either unavailable or blocking receipt of your emails then the spam protection will definitely kick in.
What you've got setup is insecure email via port 25.
Here are the talktalk email settings that you should use to update your email clients.
If you need specific guidance on a particular email client just post back.