This is probably best described through anecdote: my default send-from address in Mail on iOS is my @DavidChartier.com account. But if I type the first couple letters of an AgileBits colleague in the To field, then select the suggested contact from my address book with an @AgileBits.com address, Mail switches my From address to my Agilebits account.
I tinkered with this a bit to try and figure out what logic Mail is using, but I can’t pin it down. For example, I set my default account to iCloud, then tried drafting messages to some of the @DavidChartier.com aliases I use for things like contact forms. Mail does not automatically switch my From address to my @DavidChartier.com account, and I think it’s because none of those aliases are tied to a contact in my device.
If anyone has a better explanation of how this works, I’m all ears.[via Rune Madsen]