Going one level more formal than your correspondent is polite; going two levels more formal reads as stiff. Going one level more casual reads as warm; two levels more casual reads as unprofessional. When in doubt, scan the message you received — how do they greet you? How do they close? Mirror it.
BeforeReceived: 'Hey! Quick Q — you free Thursday?' | Your reply: 'Dear Mr. Chen, I am pleased to confirm my availability…'
AfterReceived: 'Hey! Quick Q — you free Thursday?' | Your reply: 'Hey — yes, Thursday works. Any time after 2?'