I use riseup. My current storage quota -- which I believe would be
yours, were you to sign up -- is 92 MB. Riseup says the storage
quota fluctuates a bit and might go up in the future. I think you
can also request a higher quota if you have good reasons. Since I
mostly POP everything out, the quota doesn't cause me trouble. Maybe
it would if I suddenly became famous/notorious while in an extended
coma, receiving a zillion Twitter auto-emails notifying me of new
followers. :) :(
My attachment limit is 2 MB, which is reckoned per email -- I assume
it'd be the same for you. The attachment limit presents something of
a difficulty for me, a freelance writer, since big .DOCs and .PDFs
can exceed 2 MB. But clients can deal with workarounds.
Never seen an ad on Riseup (of course, I use adblockers). I don't
know if there's a size limit for incoming attachments.
I don't think they offer premium services, but I could be wrong.
It's free with donations strongly encouraged.
One good way to donate to Riseup is through Flattr!
https://flattr.com/thing/523584/Donate-to-Riseup With Flattr you can
shoot Riseup a micro-donation via a flattr click, a recurring
subscription of flattr clicks, or just send a straightforward
donation up to your monthly Flattr donation limit, which as of this
May is EUR150.