I am not familiar with the AVG for business product or how your business setup looks like as far as email is concerned. Do those emails even make it into the server-side Inbox? I would first took into your server-side setup. Those emails could be getting intercepted (as suspicious) and placed in a special server-side folder awaiting your review before they end up in the Inbox, while they don't outright end up in the Junk folder.

Thank you for your quick response. I will look into it. I can't see any evidence of them so have contacted our internet service provider to see if they can help me.

re :I am having trouble with Thunderbird not receiving some emails.

You have not mentioned whether you are using a Pop or Imap mail account in Thunderbird.

If you have set up a Pop mail account then it can only download emails which are in the server Inbox. You would need to logon to the webmail account and check to see if emails have been sent to the server 'Spam' folder.

re:internet service provider

You may have an email address provided by your ISP; most people do. But is the problem email address one provided by your ISP or is it provided by a webhost or a free service like gmail.

Are the emails appearing in the webmail account? Are you using a Pop account? What server does the mail account use?

Thanks for responding. I have spoken to our internet service provider and it seems that the messages were being blocked by the server.