I never need emails offline on my desktop. My phone email client keeps everything but that’s just the standard Gmail app. Anytime I don’t have internet is when I am mobile and thus I have my phone and it’s far more convenient than trying to lug out my laptop or such. Also with a laptop that’s offline, I don’t see the reason for email since I already have a phone that stores emails when I need them for hotels or flights. It’s not like I am going to reply to emails during a time without the internet. Most of the time to reply I need to reference something online so even if I could queue it for sending when I do get online it’s useless to do so.