I want to keep mail on the server at about 80-90% of quota. Because when I am outside of my home, that will continue to be what I have access to. So the local copy will only be as a backup in case I delete something that I later realize I need to refer to. Since most emails are very small individually I should be able to keep the majority of them on the server. I will selectively delete either very large emails, or emails which there are so, so many of like notifications, which I will probably never need to look at.
I have used Sylpheed a bit in the past. I prefer it and a very similar project called Interlink to tbird. I just said tbird because I figured everyone would know it. But also I thought all of those were forks of tbird and wouldn’t differ much in how they work. Do they have much different internals?