I found the mail server definition file under
/usr/share/modest/provider-date/
modest-provider-data.keyfile

It contains invalid server definition for IMAP Yahoo!Mail

[yahoo.com]
Name=Yahoo! Mail
MCC=0
Domain=yahoo.com
OutgoingMailServer=smtp.mobile.mail.yahoo.com
SecureSmtp=2
IncomingMailServer=imap.n.mail.yahoo.com
IncomingSecurity=0
MailboxType=imap
***********************
SecureSmtp must be 0 !!! (no security, port:25)

Now, sending is working, only imap accessing is still problematic.