Tuta and Mailbox.org allow custom domains.

Mailbox.org is another good one.

Posteo, Mailbox.org, and Tuta are all good alternatives.

Apparently Photoshop CC 2017 has a gold rating on winedb, so that could be viable if you are willing to sail the high seas and apply the tweaks for that app listing.

Otherwise I’d stick with Krita, and then GIMP if you find Krita lacking.