Unless they are actually your colleagues (i.e. none of them are your bosses!
or clients, or third-parties), I would stick with the neutral:
Dear All, (quite common) or
maybe: Dear Everyone, (less common)
I just checked by looking up the emails we (as a group) receive from the hierarchy above: They all begin with 'Dear All' (translated in French by 'Bonjour à tous et à toutes', since the equivalent of 'all' is not gender-neutral in French.)
english.stackexchange.com/users/110/VonC
To me, Dear all conveys laziness.
A well-thought-out message should have a specific audience that the message applies to.
Dear coworkers ,
Dear minions , Dear Death-Eaters , Dear residents , etc. is not that much more difficult to type.
Then someone reading the message can more easily discern whether the message applies to them or not.
english.stackexchange.com/users/34/Gary
Dear all is perfectably acceptable.
So is Dear Colleagues.
It depends on how formal or informal you want to be, and what is normal usage in your workplace.
If in doubt, do what appears to be normal practice.
english.stackexchange.com/users/128/Evan
I find that using a non-standard, but polite greeting in an email tends to be quite effective in getting people to actually read what you are writing.
For instance, when writing to a group of people, it is quite common (in New Zealand) to write: To whom it may concern, or, as has been mentioned: Dear All,
But I tend to write something off carte like (assuming that I worked at British Telecom–which I don't):
Dear BT Person, I have found that this unusual approach—which also feels less impersonal—can be more likely to garner an audience.
But where I currently work, we have a tool to address everyone individually, we just leave a placeholder where their name is, so it will say: Dear Mr. Smith, or Dear Kevin, Which I think is probably the best way to get it done if it is convenient.
english.stackexchange.com/users/140/Vincent McNabb
Nowadays often only "All:" is used - not very nice
but that's the way it is.
english.stackexchange.com/users/43/vonjd
Salutations are not for beauty; they are a tool to alter reader's mode of understanding, often to make the message more convincing.
If you are suggesting something formal, talking from position of rights or power, you may want to use more formal "Dear Colleagues" to make the argument stronger.
If you are making a proposal, and want to downplay it as in "hey, it's an idea, a basis for further thinking and please judge it as such", a simple 'All,' would do a better job.
In all cases, basic netiquette requires that meaningless text should be avoided when addressing a large group of people.
It's easier to write than to read, so one should invest in brevity.
english.stackexchange.com/users/188/Pavel Radzivilovsky
All is not a addressing mode, when we talk to somebody we can use all to denote a group of people but while writing an email it surely doesn’t make any sense.
Dear
All is surely not correct English to start with the email, rather you could use dear colleagues at a workplace.
I presonally find writing dear
all very informal
and hence always use dear colleagues.
english.stackexchange.com/users/24891/Manav Mathue
I have observed that in my work place, whenever a mail is sent to more than one person( like an information, meeting request or a notice etc.), the mail starts with the salutation "Dear All". This, somehow, doesn't seem correct to me. For me, a salutation should address the reader individually, at least in its singular form, as in "Dear Colleagues".  Am I correct in thinking that?
Is it appropriate to use the salutation "Dear All" in a work email?
english.stackexchange.com
<email><greetings>
english.stackexchange.com/questions/292
english.stackexchange.com/users/145/Vaibhav Garg
There is some disagreement as to whether "Dear All" is acceptable or not, and some people may feel that it is lazy. However, "Dear Colleagues" would be perfectly acceptable. The answer may well depend on how formal you wish to be. Dear All is perfectly acceptable if you want a less formal salutation that something like Dear Colleagues.
There is some disagreement as to whether "Dear All" is acceptable or not, and some people may feel that it is lazy. However, "Dear Colleagues" would be perfectly acceptable. The answer may well depend on how formal you wish to be. Yes, 'Dear all' is perfectably acceptable, but something more formal might be "Dear Colleagues".
"Dear all" may be considered to be perfectly acceptable. However, it depends on how formal or informal you want to be, and what is normal usage in your workplace. Some people maintain that "Dear all" conveys laziness. The answer to this question will depend on the context and how formal you wish to be. However, even then, the answer will be subjective. Some people find it perfectly acceptable, whilst others find it lazy writing.
Dear All sounds lazy - try to be more specific. Dear all is perfectably acceptable, as is Dear Colleagues. It depends on the context - Dear All is fine but Dear Colleagues sounds more formal and proper. It is not grammatically correct, even though it is widely used.
Yes, 'Dear all' is perfectably acceptable. Dear all  conveys laziness. Something more formal might be "Dear Colleagues".
Some people find this salutation perfectly acceptable. The answer to this question will depend on the context and how formal you wish to be. For some people, "Dear All" is lazy.