Tumblr makes it seem relatively easy to use a custom domain name.
point A-record (IP address) to 66.6.44.4
But my registrar (Domain.com), doesn't make this intuitive, as there are several A Record "Hosts" that can be edited or removed:
Domain.com doesn't provide any particularly helpful troubleshooting tips here. Which is the right host(s) to modify to properly change my A record?
Check the FAQ in Domain.com's support page, under: "What is DNS Management and what DNS Records can I create and manage?"
The first record is your "Alias" Record (A Record), pointing your domain to an IP address to connect to. As instructed by Tumblr, change the IP address corresponding to your domain name there to: 66.6.44.4
I can't really see what's in the second record underneath your domain. If that's an @ or *, that generally means "anything" other that what's specified in your DNS table should go to that IP address. See "A" Record in this for more on that. You should likely change the IP there as well so that variations of your domain, such as www are sent to Tumblr's IP address too.
All the other records, referred to as CNAME records, should remain as is so that you can send and receive email, connect to FTP, access webmail, etc... on your web hosting account server's IP address.
In summary, you're just sending web requests to Tumblr, but your web host's server is handling the rest.
