I also have a Tuta account which is where I use my custom domain addresses (the number of aliases is unlimited there too, and I have so many!).
From what I’ve read, I’m not sure Posteo has a solution to this issue. I’ve read about a workaround, but also that it might have some issues.
But I do love Posteo for their stance on privacy and use of green energy, and the price is so reasonable. I’m thinking I might just have half a dozen or aliases there: one for mailing lists, one for online ordering, one for non-essential website logins and so on.











So sorry for your loss. It sounds as though you both did so much that was good for one another.