Mozilla Thunderbird [OpenSource-FREE]
It is by far the best e-mail client. It has a discrete graphics, it runs fast and has numerous functions.
Em Client [FREEMIUM]
eM Client is a full featured email client with a modern and easy to use interface.
– offers calendar, tasks, contacts and even chat.
– supports all major services including Gmail, Exchange, iCloud, and Outlook.com.
– easily imports your data from most of the other e-mail clients.
The only flaw is that it often runs slow with windows10
Probably one of the best alternatives to Thunderbird. BlueMail is a modern, mobile first, powerful Email management tool with a sleek design, unified inbox and support for all your accounts: IMAP,Exchange,POP3.
Microsoft Mail and Calendar [WIN10(free)]
It is included in windows10 but can only be used for microsoft accounts.