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.
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: 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: and protect their email address from exposure to those they don’t know. Yes, even in business 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 they don’t know.
RECIPIENTS: Even if 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 require or give you permission to email anyone who doesn’t need to know or didn’t ask for your 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.