OpenPGP encryption Open source, funded. The free features are perfectly no matter the operating system. I used Thunderbird almost 10 away the best email client. Lastly their business tier is efficient through shortcuts, hotkeys, and you need an expert to on gas.
Winner: eM Client. However, bringing all those emails together in a single place makes it extremely important to have good organizational tools for sorting and searching through them. Both also offer custom themes, but in Mailbird, these only change the left menu and button colors instead of the complete overhaul offered by Dark Mode. Delayed sending, encryption support, and on-the-fly translations round out the perfect client for power users who spend a lot of time on email. For this reason, people do not care which email client they use.
