iCloud storage full message when using mail drop

I Tried today to send a message with 2gb of attachments, with mail drop switched on in my account thinking it would not affect my iCloud storage which has 400mb free. Unfortunately this attemp has essentially frozen my mbp and I've received an email telling me my iCloud storage is full. When I go to iCloud on my ipjone or mbp it says I still have 400mb free. I cannot however receive or send emails now. Any help understanding what's going on would be much appreciated!