Emailing as a sharing option.

If you use Gmail, then you need to turn on two-factor authentication in Gmail settings and to create a password for app.
Then enter this app password instead of the password that you've used before.
Instruction on google website: https://support.google.com/accounts/answer/185833?hl=en

Go to your Google Account.
Select Security.
Under "Signing in to Google," select 2-Step Verification.
At the bottom of the page, select App passwords. (if you don't see it, then type in a search "app password")
Enter a name that helps you remember where you’ll use the app password. MixUp app, for example. Select Generate.
To enter the app password, follow the instructions on your screen. The app password is the 16-character code that generates on your device.
Select Done.

Settings for Gmail:
Port: 465
Server: smtp.gmail.com
Connection Security: SSLTLS
Authentication Method: Normal Password

If you still see SSL or Authentication error, then try using the other email. It may happen that your email was blocked by Gmail for a day or two.

Please press "Retry" to check new settings.

Possible reasons of why email sharing doesn't work:

You try to use the usual password of your email instead of the app password
You don't have internet
Your file is more than 25 Mb (for some emails it can be less)
You use not Gmail and have other port and server settings, but you haven't changed it in the app.
Recipient can't accept attachments or you mistyped
Your email is blocked for some time. Try changing it to some other.
Your Sender name is not in English.

How to set up emailing in a MixUp software?