Fax or registered letter?

No, it doesn't.

The best is recorded delivery (I think accuse de reception). That way you have a signed document of receipt.

Registered letter with accusée de réception. You fill in the papers at the Post Office, and the pink accusée de réception is stuck to the envelope and will be returned to you with the signature of your landlord.

The alternative is to actually hand the letter to your landlord AND get him/her to sign a copy acknowledging receipt (with the date).

Very simple to do pop into the post office and simply fill in a form then post, not excessive in cost and worth every penny when dealing with a landlord

Unless ... your landlord is a registered company like an SPRL or SA - then according to Article 76 of the Company code ( http://www.ejustice.just.fgov.be/cgi_loi/change_lg.pl?cn=1999050769&la=F... ) you are required to consult the Moniteur Belge to make sure you're sending to the right address; otherwise if the registered letter is lost BPost won't take responsibility for it. trust me, it happened and I am still in disbelief.