When do you use Reply to All on a mass email where numerous contacts are listed in the To: field? In most cases you don’t use Reply to All — you only reply to the Sender.
Reply to all is tricky in that there is a difference if all the contacts know each other. Or if the Sender irresponsibly added all their contacts regardless of if they know each other or not.
The last thing you want to be viewed as is breaching other’s privacy by unnecessarily exposing your contact’s email addresses to those they don’t know.
Reply to All Only if All Matters
For business emails, unless you know that your reply is necessary for everyone in the To: field to see, you need to think twice. Doing so will reflect that you understand that everyone’s time is valuable by not sending information that may not apply to them.
Senders and Reply to All
If you don’t want someone hitting Reply to All and sending their comments about your email to everyone you sent to, use the BCc: field. Unless everyone in the To: field knows each other, you need to use BCc:.
It is important to protect your contact’s email addresses from exposure to those they don’t know. And, by doing so you protect them from getting responses from your contacts that they don’t know.
This is a courtesy worth extending. If you do not make this effort, you need to acquire a taste for humble pie when your contacts email you about being emailed by those you’ve exposed them to.
Recipients and Reply to All
Even when you know some or most of the people on the list — only reply to the Sender. Because the Sender displayed all those email addresses in the To: field does not mean you follow suit.
Nor does the visibility of those addresses give you permission to email those contacts outside of that particular email thread. If you do not know these contacts, how can you assume they need to know or are interested in your specific input?
Rarely will there be a topic that your reply will be relevant to everyone involved in the conversation. If you are not sure; play it safe and reply to the Sender only.
Then, let the Sender determine if or to whom your input should be shared with.