Transitioning to iOS Apple Mail from Spark: how to get Apple Mail to recognize previous addresses?

I wanted to give Apple Mail a try on my iPhone. Up until now I've used Spark. The main problem I've encountered is that Apple Mail has no idea of any email addresses that aren't explicitly in my contacts. This is a problem if I want to write an email to someone. When I compose a new message, Spark will pull all and any email addresses that have in any way been used in prior email messages (those sent to me or those I sent), even if those email addresses aren't in my Apple contacts, and suggest those email addresses to me so I don't have to type the entire address in. Is there a way to import this "knowledge" into Apple Mail? It's a bit of a deal breaker, as I find I'm having to go back to Spark to get the email address and then back to Apple Mail to send a message. One other thing: Apple Mail is downloading ALL my old messages over many accounts. This is the reason I waited so long to try using it -- I didn't want, say, 10 years of old messages over several accounts taking up space on my phone. The size, however, seems to be minimal, so I'm giving it a try. However no matter how many times I select all the old messages in all accounts (the main heading) and say to mark them as read, Apple Mail says I still have hundreds or thousands of unread messages. I have done this over and over in the last day but it always seems there are unread messages. Is this because Apple Mail is still downloading old messages and at some point it will stop and I can then just mark everything as read? (I know I can go to each Gmail account and mark them as read there, but that is somewhat time-consuming -- the nice thing about Apple Mail is that it lets you interact with multiple accounts at once).