Several family members have domains (mostly at 123-Reg) and use email forwarding for xxx@<their domain> to their personal addresses (eg at Gmail, Virgin, BT, whatever).
Increasingly they're finding that mail sent *from* their domain addresses is ending in recipient junk folders.
How they send varies: my dad used Thunderbird on Ubuntu (so maybe not entirely OT!), my wife sends via Gmail, others I'm not sure but probably thinks like Outlook/Outlook Express (or whatever it is these days). They'll be using the SMTP settings of their ISP or personal mail provider (eg my wife will be sending via Gmail using her own domain set up as an alias).
Since I have several people with issues - and especially since one of them is my wife! - I want to take a fresh look at all of this and get things right. I'm guessing that this is going to involve SPF, DKIM, DMARC stuff that I don't really understand but have succeeded in setting up before.
It feels like something that must be a sufficiently common issue that there would be plenty of help out there but it's one of those issues where I can't see the wood for the trees in any search I try, so I'm hoping someone here might have some pointers?
The more that can be done server-side the better as that reduces the need for me to help lots of people configure lots of different clients. If I need to move domains from 123-Reg that's not out of the question - indeed I have plans to do that to somewhere I can control DNS, mail forwarding, etc via APIs anyway.