What is your opinion on using "Dear All," in business emails when addressing to a group of people? Is this too informal? Or has it been an accepted usage?

The situation is that I know all the recipients but I can't find a simple term that can represent all of them. The recipients include my colleagues and external parties. There are a number of alternatives but I think none of them fits in the situation.

Dear Sirs - sexist
Dear Ladies and Gentlemen - seems too formal and outdated
Dear Colleagues - not all of them are colleagues, some are external parties
Dear (name of committee, etc.) Members - not all of them are members

Do you have any other suggestions?

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Comments  (Page 6) 
Dear All
"Dear all" is absolutely conventional and correct usage for the scenario described by the poster. It is appropriately formal for an e-mail in business English. "Hi everyone" is indisputably less formal! So you might use "Dear all" for (say) a Department-wide e-mail or a relatively formal e-mail to colleagues and external parties; and "Hi everyone" to update perhaps a smaller team of colleagues whom you know better. Also the former might be better for an e-mail of greater importance (eg re data security) and the latter for organising a team social.
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I can see 50-50 in the response, however how does it make difference if you mention "Dear All" in your business email and is there any written or rule to say so? kindly share the knowledge. I dont see any issues and it was acceptable

I think the best policy is to see what your clients and colleagues do, and then take the same approach.