Sir,

Dear woman

Madam, a slightly more formal company will send emails to its employees by name, and each letter will have a signature at the bottom. If found, use Mr XXX.

or

lady

XXX. It may be helpful to look at the e-mail address. Finally, if both men and women don't know, they can only use dear.

Friend, but it's not good to write a letter without a name in English.