Q: sending messages with ipad impossible:user name or password incorrect

Hello mipage,

For that error you would need to make sure that the password is correct in Settings > Mail, Contacts, Calendars and look at the outgoing server settings. If you are unsure of what it should be, then the easiest way is to remove the email account and add it back in to add the correct settings. For more information, take a look at the article below 

Get help with Mail on iPhone, iPad, and iPod touch

https://support.apple.com/en-us/TS3899

Regards,

-Norm G.