With slowly but steadily growing desperation I spent three days trying to get email working somehow. Started with Postfix, the program that can receive and send e-mail, say: the administrator of the letterbox. And then Dovecot, the program that can save the e-mail and offer it to end users, say: the administrator of the doormat ;-). As it says somewhere in a manual: Postfix is ​​pure misery because of all those settings, complex or small, that you have to assess and possibly correct. And if you think Postfix is ​​difficult then you haven't seen Dovecot yet, because setting up Postfix is ​​a paradise experience compared to Dovecot. This combo has basically become an industry standard, so it's obvious to try it first.
Probably purely due to lack of patience I stumble over it. The emails swirl around me and I get nowhere. And it's with mail just like with flour: if you have too much of it, it can explode! (In Dutch this is linguistically a nice construction. The Dutch word for flour is meel, which has the same pronunciation as mail.) For now, I'll give up the duo P&D and I'll keep looking...

Tried various program packages. Some are purely a shell that installs Postfix and Dovecot or alternatives to them; others are written from the bottom up as a mail server. But nothing works well. The ultimate test is and remains to write an email to myself with my mail program (Thunderbird). Then you test sending, receiving, and internal transfer. But nothing works well and most of it already stalls when sending.

I got Dovecot to work on my home server, but it works very differently. Because Postfix or something like that ran on Robohost's servers, I only had to fetch the emails with the Getmail program and transfer them to Dovecot. I cannot copy that Dovecot setup to the VPS with impunity because the folder layout and user names are different. That can be figured out and it would solve half the problem. But without a solution for Postfix, I can't really use that: it's still as if someone has taped the letterbox!

But after three days of struggling, another solution pops into my head. When I spent a few hours trying to understand Plesk, I saw that not only can you define domains and attach websites to them, but you can also manage email. Some reading in Plesk manuals tells me that Plesk is also a shell from which Postfix and Dovecot are installed and managed. That's probably exactly what I'm looking for and then I'll have to take the disadvantages of Plesk into the bargain.

First, check if I can migrate the migrated websites again.... that's possible, I still have all the files. So... started all over again. Created another installation order for the VPS, this time again with Plesk. It seems to take a while and I'm too tired, so off to bed, and we'll see tomorrow.