Recently a site I was hosting on my shared host decided to move their online presence off Joomla to the shopify platform. The website was originally on a shared host; which is fine but when the site was moved to shopify, I was asked to update the A and cname records for the domain to the shopify ip and url.
My host on CPANEL when you setup a domain, puts in a bunch of default settings for the domain, as they should. Here is a sample of a default setup.
This client is forwarding domain emails to a gmail account, they don't have google apps or anything like that and it isn't required for them. So when I updated one mx record pointing to my main a record and when I updated the A record to point to the new IP and change the cnames to point to shops.myshopify.com, my client being unaware that shopify doesn't provided email service email just stopped working because the mx record was pointing to the main A record. This caused all email pointing to domain.ca to stop which was being forwarded by the server to a web email service.
The solution was to make a new A record with the old ip address in this case making a name record mail.domain.ca A = sharedhost IP and putting a new cname domain.ca MX = mail.domain.ca
Now you can see here I removed the mail.domain.ca cname record I felt it was redundant but if I have problems I can always put it back in especially since the mail.domain.ca was not being used. I do have to setup DMARC but that is something else that I'm not doing at the moment.