Even better than looks however, is Yahoo Mail's new smarts. Take the new contacts card, for example. When you tap on a person's photo, you'll immediately see their contact details even if he or she is not in your local address book. That's because they're all automatically culled from their connected Facebook, LinkedIn and Twitter profiles, and even from their email signatures if they contain alternate email addresses or phone numbers. The contacts card also helpfully shows the history of email exchanges between the two of you and a list of what it thinks are related contacts (perhaps mutual friends or co-workers). The latter, Delgado says, is especially helpful if you're trying to remember who the person is and how you met them.

More powerful than that is Yahoo Mail's brand new contextual smart search engine. Type in the first letters of someone's name, and it'll instantly suggest the most likely contact. For example, say you entered in your co-worker's name in the recipient field and then proceeded to type in a couple more letters. It'll automatically suggest the name of other co-workers you've usually emailed together -- a feature that's been on Gmail for awhile now. What's impressive with Yahoo Mail's version though, is that it's even more contextual than that. Say you typed in your co-worker's personal email address instead because you want to organize a non-work-related movie night. The next couple of letters will bring up a list of people with personal email addresses rather than their work address. "If the user has to type in more than a couple characters, we haven't done our job," says Bonforte. "It's these little things that when users get used to them, they sort of feel blind without them."

Ah, but that's not all. Say you're doing a search for messages from a particular friend. While traditional mail clients typically only fetch what's locally on the device or the top 100 or so messages on the server, Yahoo Mail will search your entire inbox even from months or years ago. And yes, it'll search through all those multiple third-party accounts too. That's because the search is server-side powered, not client-side. It's not just messages either; you'll be able to see all the images and files your friend has ever sent you, and all the images and files that you've ever sent to them.

"We think we have the most advanced search in a mobile email client out there," says Delgado. "We truly understand the structure of the messages, the files and the photos that are in your inbox."

There are a few other neat features with composing new messages. You can easily include multiple photos from your camera roll to a single message and get them to render in line by default, or you can attach them like traditional attachments by hitting the plus sign at the bottom. For the Android app, you can also browse your phone's system folders or your SD card to find the file or image you want to attach. But what's particularly powerful here is that because Yahoo Mail indexes every single message that you've ever received or sent, you can also easily search for and attach any photo you've received without having to download it and attach it -- all attachments are done server-side, even if it's an 80MB file.

If you often use your email to send yourself reminders, you can simply long-press the compose button and your own email address will automatically appear in the To field and the cursor will be the subject line. Type the note, hit send and you'll get those reminders. "About 20 percent of our users frequently email themselves," says Bonforte. "It's also aware of all your email addresses ... emailing yourself from any one of the accounts will be considered an email to yourself."

And if you have the new iPhone 6s or 6s Plus, you'll be able to 3D Touch the app icon to "peek" into the app and compose a new message, send a note to yourself, conduct a search or go straight into the inbox. For those with iOS9, you'll be able to search for anyone on Yahoo Mail right from Spotlight. Tap on any of the Spotlight results, and it'll switch to the app and email message in question.

Of course, Yahoo Mail has a steep hill to climb in order to defeat the juggernaut that is Gmail, but with innovations like Account Key, the willingness to support multiple third-party accounts, and its huge server-side Smart search chops, the new Yahoo Mail is certainly worth a look. The new app should be available on both Android and iOS today, while you're able to add Account Key to your account starting today as well.