Ok I’ve found the solution 
In Snappymail admin panel :
Domains > *
Edit Domain "*" > IMAP, set Secure to SSL/TLS (it was None by default)
Edit Domain "*" > SMTP, set Secure to STARTTLS (it was None by default)
No more auth problem when sending mails
(I think the setting for SMTP is enough, but I think setting it up too for IMAP is not a bad thing )