Some of my users use an external SMTP gateway (such that when they send email, it doesn't go through smtp.gmail.com).
I have an SPF record configured for the domain, and this allows the external SMTP gateways as valid SMTP hosts. (I realise SPF is different to DKIM)
Will enabling DKIM adversely affect the external gateway email? eg. Are the externally sent emails at risk of being marked as spam because they would not have the DKIM signature, or will DKIM only positively benefit emails sent through Google's SMTP server?