I have registered a .com domain and received an e-mail from domainadmin.com

As Polynomial mentioned, this is part of ICANN-mandated WHOIS verification. The reason it goes to domainadmin.com is that ICANN doesn't actually run the verification -- rather, like just about all ICANN things, they set policies that are then implemented by others (remember, your .com domain is in a registry operated by Verisign, and was registered by a registrar who was neither ICANN nor Verisign). It's the registrar's job to verify certain WHOIS info; the relevant policy is here, and they must suspend your domain if they can't verify the info.

The reason you're seeing domainadmin.com is that whoever you bought your domain from is likely reselling them from OpenSRS (which basically exists as a platform for domain resellers), which is a label of Tucows. As the registrar, Tucows is responsible for WHOIS verification on domains bought from them; because OpenSRS is mostly a platform for resellers, they generally do so via domainadmin.com (which they own, and which is intentionally white-label). Source 1, and a discussion on their site about this domain choice.

So, this email is likely legitimate, and you need to do what it says or your domain will be suspended. domainadmin.com isn't affiliated with ICANN, it's affiliated with your registrar (you actually bought your domain from a reseller of this registrar). It's intentionally generic because some resellers want a generic thing; resellers who want non-generic have the option to make it non-generic, but most resellers don't want it showing OpenSRS or Tucows prominently.

It's legitimate. It's part of ICANN's registration process for certain domain providers.

References:

• http://pgregg.com/blog/2014/06/domainadmin-com-phishing-spam-email-that-isnt/
• http://www.scamadviser.com/is-domainadmin.com-a-fake-site.html
• http://www.ukfast.co.uk/blog/2014/01/21/how-do-the-new-guidelines-for-registering-domains-affect-you/
• http://supportem.com/blog/1410/icann-whois-validation-process/