Why can't I download e-mail content?

I'm using the Email app on my Samsung Galaxy S8. Recently, I've noticed that, on occasion, when I tap on an e-mail to view, the e-mail content doesn't download and I continuously get that right-sweeping thin blue horizontal line at the top of the e-mail. It tends to happen (I think) when the body of the e-mail contains HTML content, and perhaps more crucially (but again I'm not sure) when that content includes an internet link. A workaround is to proceed as if to forward the e-mail (without actually forwarding it), whereupon the e-mail content to be forwarded becomes visible, although in this instance, with this content being "editable", there is not the ability to tap on any link there included in the content to direct you accordingly.

Please note that this enquiry is in no way about e-mail attachments.

Hope somebody is able to help!