非常感谢 is the go-to 'thank you very much' — notably stronger than plain
谢谢 but natural in both spoken and written contexts. For the heaviest gratitude, escalate to
万分感谢 (wànfēn gǎnxiè, 'ten-thousand-fold grateful'). The emailer's classic sign-off is 'thanks in advance':
先行感谢.
