I have windows 10 and when I reply to an email or start a new email the program crashes when I start a new line.

Program crashes every time I hit on a reply or new mail entry.

Could be corruption in your database.  My suggestion is to delete the database and reenter your account information.  Assuming you are using IMAP or Exchange, you shouldn’t loose any data.  To do this–

1.  Close eM Client.
2.  Navigate to c:\users[user_name]\Appdata\Roaming\eM Client and change the name of this folder (note:  it is a hidden folder, so make sure you have the show hidden files option enabled in Windows explorer).
3.  Restart eM Client.  The program will create a new database.
4.  Reenter account information.

The program will then resync your account(s).

I’ve followed those same instructions before and only fixed the issue for a few days then it began to crash again, and more frequently. I gave up on EM client.  I’ve been dealing with it for over a year, and when it crashes it takes an hour for it to go through the system check before I can reopen the emClient. GOOD LUCK.  I like the user interface, I hope they find a permanent fix for this issue soon.