How do you address your boss in official emails?

Lets say my boss's name is Omar kazaq

Only in serious formal emails will I use his last name such as a resigning/complaint letter

For example; Dear Mr. Kazaq,

Regarding the less formal letters, holiday requests,etc.. I would use his first name

For example; Dear Omar/Sir, (Refer to them as you would on the phone)

Most bosses are making decisions the whole time which means their brain is always full of things to think about ; So when sending the e-mail make sure you mention a simple clue in the subject to benefit your boss rather than goodwill and effort from him.

Moreover, business texts should not be mystery novels + never give default options to your boss,instead you may ask what he/she intends to do.

Thanks for invitation,

Agree with the answer of Mr. Omar Saad, just may I add:

- To add a "clear subject".

- Be clear, brief, direct and precise.

- Marking your e-mail ( i.e. private, personal,........). 

It is quite normal to write official emails to your bosses. You can follow his style but don't get too informal or chatty yourself. Note that some bosses intentionally write short, informal messages but expect a properly articulated, grammatical response.

The tricky part is if you want to complain about your coworkers, even if the complaint is legitimate. My advice is to keep such matters away from emails that may get cc’ed all over the office. You could land in a lot of shit.

it is always better to address to any official with DEAR SIR/ MA'AM 

Depends upon the culture of your organization. You can start "Dear Mr. XYZ" or "Dear Sir". 

There is one way to address your boss and that is always the formal way. Starting by dear Mr., Dr, sir... and with short and very detailed phrases that describe the reason of any email. Unless your boss has asked to be addressed less formally for certain issues, the rule is being formal, be detailed and be compact, unless asked the opposite. Keep in mind that most of the time there is at least one person higher in hierarchy that monitors the emails. If the email is not sent by office servers and the relationship with the boss is more friendly then be friendly but only then. 

Thanks

In simple steps, you need to follow the next :

• Do not put his-her e-mail before you finishing the content and review it twice.
• If you have to attache something i.e. Picture, PDF, Documents, Charts, do not forget to do so before sending your e-mail, since if you send it without it that will be so embarrassing.
• Be sure that you involved the related person on the e-mail loop, or do not put anyone (just your manager if you want to send it personaly)
• Start your e-mail by (Dear, Dear Sir, Dear Mrs, or Dear Gents if you are sending your e-mail to your manager and others..) do not use or start with Hi, Hello, Good morning,...)
• Do not forget your official signature, or end your text by Regards, Best Regards, sometimes Thanks or Many Thanks if you want to ask him-her something.
• Finally, Put his-her e-mail address and send the e-mail

Regards