There’s useful discussions at the link:
I’m hosting Sympl on a MB vm. I purchase domains via 123 Reg and have resorted to adding the spf, dkim and dmarc settings to 123 -Reg’s dns servers which works fine. Some crystal clear Sympl documentation would be a dream. I find myself fishing for detail between Mythic and the Sympl Wiki. I failed to establish the dns setup correctly via MB so I decided to cheat and use 123 Reg’s dns servers ie circumvent the problem!
This is some really useful discussion here: Email deliverability question
Noteworthy - I hadn’t ever realised the importance of adding the server host’s name to spf records for domains being hosted. Every day’s a school day!