configurar wp mail smtp (error 500)

Buenas.
He configurado el plugin wp mail smtp con mi cuenta de gmail con los datos correctos (smtp.gmail.com, puerto 465, ssl…)
Al hacer el test recibo un error 500:
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at admin@main-hosting.eu to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.

Alguna sugerencia?

Gracias.