Can't delete email with large attachments

I often receive emails with large attachments (5-15MB). When I try delete them in eM Client, they immediately re-appear in my email list. If I go to my mail providers website client and delete from there - no problem. Any thoughts on why eM Client won’t delete these emails?

I am assuming that the account is setup as IMAP, but it would be good to know who the email provider is.

Indeed it is setup as IMAP - provider is Shaw Communications (Calgary, AB, Canada)