ICloud Mail and BT Internet

Hi,

I have an iCloud account as well as BT Broadband with @btinternet.com email addresses. The BT email is set up as IMAP with a mail client on my iPad, iPhone and IMac. My partner also has a btinternet email address and if she sends or forwards an email to my iCloud mail it never arrives and nothing in junk either. The same thing happens from my own btinternet address.

However if use the BT webmail and send to my ICloud Mail it arrives.

Both BT internet mail accounts are set up with the correct settings and can send/receive elsewhere without a problem. So what is different between an webmail email and an imap client email that stops the imap arriving. Is there some setting that needs changing or adding.

thanks

using iPad Air iPadOS 13.3 or OS X Mojave