Best email client for Outlook, Yahoo, and Gmail?

I’ve got a bunch of different accounts scattered across Outlook, Yahoo, and Gmail, and I’m really itching to just pull them all into one single email client so I can stop jumping between tabs.
I gave "Mozilla Thunderbird" and "Pop Peeper" a shot, and honestly, the Outlook stuff works fine, but whenever I try to send anything through my Gmail account, I hit this annoying error message—even though incoming mail comes through without a hitch.
It’s weird because I followed the setup instructions to a T, making sure all the IMAP settings were dialed in correctly.
Does anyone know of a different client out there that actually plays nice with everything and won't give me a headache?

If you're running Windows 10, you really shouldn't have any issues using the built-in email app. It’s right there.

Sent from my iPhone 7 via Tapatalk

Unfortunately, I'm still stuck using Windows 7

I guess Microsoft Outlook works fine for me. Haven't really had any issues with Gmail.