When using third party tools which send mails from your domain, you often need to setup DKIM records with selectors defined by the third party. E.g. for MailChimp you're asked to setup:
The selectors (
k3) must match those defined by the third party, in order to function as expected / match the selectors sent in the mail's headers.
According to this answer, DKIM selectors must also be unique; i.e. you cannot have multiple public keys associated with the same DKIM selector (so whilst DNS would allow me to create 2 TXT records called
k2._domainkey.example.com; that would lead to unexpected functionality; most likely intermittent mail failures.
However, there's no rule to ensure that the third parties provide unique selectors; or even standards to make collisions less likely (i.e. MailChimp don't suggest using
mailchimp1._domainkey.example.com or anything like that); most examples of DKIM records I've seen have a 3 char selector (typically 2 characters and a number), giving only a few thousand unique selectors; so collisions are reasonably likely.
Note: I mention MailChimp above as that's a well known example mail provider; but this is a generic question, rather than specific to that provider.
What options are available when DKIM selector collisions occur?
Is it possible to satisfy both provider's requirements, or is that only possible where the providers have added functionality to allow you to specify your own selector value.