hi Jerome do you have idea how 

customize elgg style and integration of smtp and using ldap

For customizing the Elgg style

you can use the basic theme settings on /admin/theme or if you need more a custom plugin needs to be developed.

SMTP integration

You can set the smtp settings in /elgg-config/settings.php

Using LDAP

I take it you wish to use that for authentication. You need a custom plugin for that. I don't remember if there are plugins available, but search is your friend.

If your authentication platform supports SAML you can use https://github.com/ColdTrick/saml_sso