Yenthe,

I have asked same question before few days :

https://www.odoo.com/forum/help-1/question/how-to-send-email-by-userwise-modelwise-using-differnent-outgoing-servers-87768

I made a module for this and added a many2one field in outgoing server ( ir.mail_server model ) At the time of send mail, i checked if there is no default mail_server_id available then system will get the related mail server from "uid".

For example, see following method. ( Please give attention to bold lines )

class ir_mail_server(osv.osv):
    _inherit = "ir.mail_server"
    _columns = {
                'user_id_ept':fields.many2one('res.users','User',help='When user is assigned then \
                                    at the time of email sending from Odoo if no outgoing server found then \
                                    system will take the server which is assigned to this user.') 
                }
    
    def send_email(self, cr, uid, message, mail_server_id=None, smtp_server=None, smtp_port=None,
                   smtp_user=None, smtp_password=None, smtp_encryption=None, smtp_debug=False,
                   context=None):
        mail_server = mail_server_id
        if not mail_server :
            user_obj = self.pool.get('res.users').browse(cr,uid,uid)
            email = user_obj.partner_id and user_obj.partner_id.email or False
            if email :
                mail_server_ids = self.search(cr, SUPERUSER_ID, [('user_id_ept','=',uid)], order='sequence', limit=1)
                if mail_server_ids :
                    mail_server = mail_server_ids[0]
        return super(ir_mail_server,self).send_email(cr, uid, message,
                                                    mail_server_id=mail_server,
                                                    context=context)  

In a nutshell, before Odoo checks low priority outgoing server in case of if there is no mail server specified, I added condition to check outgoing server related to user configured on it with "uid". ( this sentence may be hard to understand ;) )

From my thinking this is the only way. However in my question i raised another interesting point. If you have a time please read it as well.

Latest Update : This module is out to Odoo store now. It is a Free App. You can download from this link.