Posterous recently implemented simple Custom Domains. So I decided to try it. But Posterous's help file doesn't actually help. First of you don't need to change the A record. What you want to do is change the CNAME instead to posterous.com. If you look at the following pics, it'll all make sense:
Log into your admin account at 1and1, and then get to Manage Domains.
Then create your domain, in my case I made a sub-domain of reverenddak. There is probably a bunch of things you have to do if you're doing a new domain. I didn't try. But it's probably easy for you if you work on websites.
Next you select the domain you created.
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This is where Posterous's help doesn't work for 1and1. You don't want to change the A Record, you need to change the CNAME. And you enter posterous.com as the Alias. Hit OK or whatever, then you're done on the 1and1 side. You probably should get a coffee, or catch up on a TV show, because it might take a while for it to take effect. Sometimes it is immediate, sometimes it takes a while. I've done a lot of subdomains, and it's pretty typical. Â
 Ok, when you come back to your computer, log into your Posterous account and go to the Settings page for your Posterous site. It can only be your primary account, well it is for me. Edit or modify your site. Then enter the domain your created under domain. When you hit save it should be done.Â
So go and check! It should work, if it doesn't I can try and help, but yeah…
So go and check! It should work, if it doesn't I can try and help, but yeah…I hope this helps some of you. You'll also notice that your old URLs will automatically redirect to your new fancy domain. All old URLS on other sites that pointed to any of your posts will still work. It's pretty cool.Â