If there are two of something, can you say "all of" instead of "both of"?
For example, if an email is addressed to two people, and you start off with:
This is to inform you, etc., etc.
Is that at all appropriate?
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I would say use "All" if you have three or more. If there are only two addressees, just name them.
"To: Dick@home.com, Jane@home.com
Re: Doghouse plans
Dear Dick and Jane,
Please let me know if you have reviewed the plans for Spot's new doghouse. The contractor will be arriving Monday and we need to have any changes settled before then.
"To: Dick@home.com, Jane@home.com, Dad@home.com, Mom@home.com
Re: Doghouse plans
Not to hurry you or anything, but it has been raining for the last three days, and Grandmom has been complaining about my bivouacking under the kitchen table. The sooner we can get this thing built, the faster you can get me out of the house so I'm not stinking up the kitchen with the smell of wet dog.
Bark bark, Spot"
Given that email typically takes on a conversational tone, sure. If the email (or letter) is more formal, perhaps use an alternative.
The example usage (an email sent to just two people) is probably a bad choice. Most recipients (if they could see that only one other person was cc'd in) would probably feel flattered.
Assuming the sender was competent in the English language, the only logical explanation would be that the writer had originally intended to address several others - but before clicking on "Send", had decided that only our flattered recipient and one other were worthy of being communicated with.
Outside of certain dialectal contexts where you all / y'all implies plural (largely because we no longer use "thou" for singular), "all" always implies more than two (as does several).
* denotes "marked" (i.e. - non-standard, unacceptable) usage.