There may be some cases where a third party email provider will not have support for DKIM. There are a few options available to workaround this.

Option 1. Ask the email provider if they can SMTP relay those emails through another service of yours that does support DKIM ie. ServiceNow relaying the emails via Mimecast. In this case the emails will pass through Mimecast and be DKIM signed by the gateway on the way out. 

Option 2.  Work with your email provider to introduce support for DKIM or ask for an ETA.

Option 3. Separate the traffic for that email provider into a subdomain until they introduce DKIM support. In this case your main domain will support SPF and DKIM while the subdomain will only support SPF. Any failures in the subdomain will not affect your main domain’s traffic or reputation.

Option 4. Migrate to another email provider that supports both SPF and DKIM. And in the future consider evaluating providers that support both SPF and in particular DKIM so that we do 

Option 5. If you have no existing gateways or SMTP servers through which to relay the emails from the SPF-only aware sources, then you could sign up to a service such as SMTP2Go, Sendgrid or Mailgun and relay the emails through them for a small investment. It could also be free depending on the email volume that you will be relaying.

Option 6. Use the default domain of the email service in question, but change the Display name to be your Company's name. 

For example, Shopify supports only SPF and they are using your domain in the From address such as <sales@your-company-domain.com> . If none of the previous options listed above work for you, then you could ask Shopify to change the From address to sales@shopify.com but use a Display name to match your company ie. "Company Name" <sales@shopify.com>
This is a good workaround as your users / recipients will see your company name in the Display name of the emails. This will also let you continue working on protecting your domain with DMARC as the Shopify emails will no longer be associated with your domain. 

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