Use dear sir/madam directly, and then you can write yours sincerely at the end.

Dear Zhang San:

******

Kind (best) greetings.

(Name & Address)

And reply emails are usually marked with (sir) or (madam) after the name. If you need to continue to reply to this email, you can use Dearsier or DeardMadam directly according to the title.

Dear XXX

I have difficulty logging in. The account has been disabled. My username is XXX. Thank you for your concern and hope that the problem can be solved as soon as possible.

send one's regards to

XXX

If you can't remember your password, you can find something similar to "Forgot your password" on the school website after your account is activated. Just fill in some basic information and a spare mailbox, and the password will be sent to your spare mailbox.