I don’t know why the full mail isn’t shown, works for me. What browser is this (maybe this is related? Open mail in right with but wrong height · Issue #5868 · nextcloud/mail · GitHub)? The borders are a feature not a bug, I guess. But you can hide them with CustomCSS:
@media print
{
.envelope {box-shadow:none !important;}
}
The sitezoom doesn’t have any impact on the printpreview. The thing with the open menu should be a bug, maybe you wan’t to write an issue to the developers? Issues · nextcloud/mail · GitHub