how can I get mail in Santiago?

You can have it addressed to yourself at the post office (Correo) in Santiago. If you are sending it go into any Correo on the route and they will tell you what to do and will confirm the address where you wish to send it.

If someone else is sending it tell them to post it to You (your name) at the post office near the cathedral in Santiago which is:

Your Name
Lista de Correos
Travesa de Fonseca,
15705
Santiago de Compostela
Spain

However the Post Office will only guarantee to keep it for you for 15 days.

If this is simply a piece of mail and not a package you can have it sent to the Pilgrims Office and they will keep it for you:

Rúa do Vilar, 1, 1º.
15705 SANTIAGO DE COMPOSTELA
A Coruña - España

However I must stress that this is a courtesy service for pilgrims and the Pilgrims' Office accepts no responsibility for the mail which is delivered for pilgrims. It is not suitable for valuable or vitally important documents,

You may also consider having it sent to the hotel or hostal where you are going to stay in Santiago.

I hope this helps.

John

I was told that, unlike elsewhere in Spain where there is a two week limit, at Santiago the post office will hold mail and packages for 4 weeks. We did this and had no problem, sending things on for just on a month in advance.

The correo guarantees to hold mail for 15 days only.