So I run my own email server for family (mostly) but a couple of friends. As the GP points out, the DKIM keys do not identify the user and in particular I can produce a validly signed email for any user of any of the domains my server hosts. Only CMS oblique S/MIME would provide non-repudiation in the cryptographic sense, and only meet e.g. eIDAS requirements if stored on a hardware token (i.e. in the EU, if I sign a PDF with the USB token I have, it is a "qualified signature" and it can be interpreted as legally binding).
On the other hand, there's a question as to what extent courts would understand the distinction between a valid and invalid/no DKIM signature. I guess what I'm saying is if it came down to a contractual dispute, "strong evidence" would not be good enough. If it came down to "this person organised a protest and that's a crime" I think even invalid DKIM would be fine. From what I've seen of court proceedings, mostly stuff shared here and so US stuff and not my native jurisdiction, but I have yet to see anyone cast doubt on an email's provenance based on DKIM. I'm going to go out on a limb and say in strongly democratic jurisdictions an email recovered from a server in someone else's inbox will likely be treated as valid unless there is something like an unusual sender IP to cast doubt on it. So "strong evidence of authenticity" probably won't come from DKIM.
In jurisdictions where protesting means you disappear, I doubt "check the DKIM keys!!!" is going to help - at least in part the police/courts might not be so worried about whether you are guilty or not.
Agree completely with the sibling comment that e-mail isn't suitable for secure messaging.