This week, I've recently started receiving a "sender denied" response whenever I try to send an email. At first, I thought it was because I'd tried to send an email to a list of about 20 people. But I now get it every time I try to send something -- even to another email address that I use.
I have two TALKTALK (well, Tiscali anyway) email addresses, and this happens on both of my addresses. It happens both when I use a desktop, and when I use my iPad.
There are a few reasons for the sender denied message. Sight of the full error message would help. For example: 554 5.2.0 Sender Denied (TT608).
The first reason is that your group email may have invalid email addresses or recipients are blocking receipt. You'll need to check these addresses one by one to make sure recipients do wish to receive your emails.
The second reason is that multiple logins to your email account have caused the system to believe the email account may be compromised and used to send spam messages. The easiest way to fix this is to change your email password after first scanning your device to prove it's not harbouring viruses or malware like keyloggers obtaining passwords.