Sent the wrong instructions to a client and they changed their nameservers to Digital Ocean instead of creating a new A-Record.
It was 100% my fault as I had not seen they had previously added the needed MX-Records to their GoDaddy account.
Seems you now only need a single smtp.google.com MX-Record; this is a far cry from having to enter 5 MX records like I remember – it still works, but is the “old way” now. [1]
Added the MX-Record to the Digital Ocean Networking->Domains->[Domain Name] DNS records. [2]
Tested the email sending and delivery with the client and all was well.