There is a welcome addition to El Cap that I like a lot: in Mail you can finally create and edit mail aliases from the UI. This was previously possible if you edited
~/Library/Mail/V2/MailData/Accounts.plist file and added your aliases as dictionary items (I blogged about this in detail 2 years ago).
Why is this useful? If you have custom domains that you’d like to send mails from using your current mail provider as a backend, you can set aliases for that domain. Apple Mail will recognize the recipient list and automatically set that address as the sender address.
For instance, I have a reply from address set to this blog, and whenever get a mail from a reader, I’m automatically replying from this domain. Apple Mail even lets you quickly select from your identities in every mail (just like as if you’d had multiple mail accounts configured).
One thing to note is that you need to provide domain ownership, otherwise your mail will definitely end up being in the spam. This can get technical and messy, but refer to my earlier blog post or reserch DKIM and SPF zones.