@Paris Geek: Exactly!
E.g. I have a work account that has probably over 50 000 messages. If Aquamail had to look through all those messages each time I'd need to send a new message, that would take some substantial time (not to mention the associated with that data usage).
So, to make it efficient, an app can cache/accumulate all those addresses. Where? One possibility is in the cloud. (And that's what those apps probably do, I assume.) Another possibility is to store as a local database. That's why Aquamail is offering an option to store all the addresses of your e-mail recipients into "Contacts" automatically.
I understand @nibynoga's hesitation about contaminating Contacts with all those addresses (or even to save some significant number of them manually). I personally have two reasons:
1. I don't want to dilute my phone contact book (which I use for phone numbers primarily), and
2. I don't want all the apps that are snooping into Contacts to collect all those e-mail addresses. (I don't want other apps to be able to snoop in my Contacts unless I explicitly allow that, but that's a different battle.)
As a matter of fact, I would have preferred the e-mail app to have its own address book (in addition to be able to access "Contacts" database). IIRC, I expressed this to Kostya about 2 years ago, and (again, IIRC), Kostya saw some rationale in it, so it might even be somewhere on his list. But I can see that it would not be a high-priority feature, compared, say, to the recent EWS-Push.
But if more people like @nibynoga and myself would voice their need for that, that might move it up on that list (or place it there, if it is not there yet).
