Apple Mail - cannot view emails

Hello

can someone help with email? I have a really strange situation with viewing my emails. My mail account is IMAP and I access it via my iPad and iMac. I tend to read my emails on my iPad first and then at a later stage access them via my iMac for actioning eg - filing, paying bills etc.

The problem is that if I receive a large email (usually the attachment), I cannot view it via the iMac and cannot view any subsequent emails. Apple Mail on the iMac shows 'downloading messages' and the number of new messages. As I watch the screen the number of new messages can change wildly but the email list does not update.

If I access my emails via web mail and delete the one with the large attachment, all of the subsequent ones will then become viewable on the iMac.

This is a major issue as I use multiple mail accounts and do not know if I am 'missing' any emails unless I carefully check the received dates and notice that days are missing. This is not ideal.

I have no idea what to do as I need to access all my emails seemlessly on both devices. I have contacted my ISP who says it's an Apple issue hence I am posting a question here.

Can anyone shed any light on this issue and how to fix it?

Thanks