Hello Tom, do you like to send us a ticket with the error message and your SPF record plus the website ID? This might be related to this:
…and not DKIM.
I like to answer a few of the questions you raised:
Would implementing DKIM capability in Sitejet be a major project requiring a large amount of resources?
Yes. And with the integration to Plesk for example you are able to use Sitejet and set up your own mail server as well.
Is the intent to make Sitejet less attractive to use for email and shift users email activity to some external provider for some reason?
No, there is no intent and the current settings already discussed in this article are fulfilling the basic needs.
I’m just trying to understand the logic behind the multi-year decisions to not pursue DKIM in Sitejet. Especially now when Gmail is requiring it – Gmail has over 30% of global email market share and over 50% market share in the US. Not allowing compliance with their standard seems strange from a business perspective.
I like to point to the other information we have posted here and the thread I linked above that should make it more clear that right now this is not needed for the basic email features.
Hope this helps.