On your domains DNS settings, you cannot have two identical HOSTNAME records.
Problem:
I ran into an issue with one domain where I was using MailGun for email forwarding and SMTP setting as well as MailChimp for newsletters and email automation. In order to have your emails properly delivered, you need to have a CNAME record for all platforms. MailGun assigns the DKIM selector of k1._domainkey and MailChimp assigns the same DKIM selector k1._domainkey so they both cannot exist at the same time.
The only solution is to have one of those providers change the DKIM selector for you. MailGun responded to my support email in less than an hour and changed the selector to mailo._domainkey so that I could use the default k1._domainkey for MailChimp.
Solution:
Contact one of your services to change the default DKIM selector for you.
