How bad is this? I send out notifications that projects are completed to customers. I sent out an email that said "The Blah Blah Blah for Company A is complete" and got the response "I work for Company B, I think this got sent to the wrong person." I felt mortified. The person had replied to everyone(this includes my boss and internal consultant). I promptly hit reply all, acknowledged the mistake, and apologized. This might sound like no big deal but my work involves dealing with sensitive information and I'd hate to think that this customer now feels her information could now be sent to someone else. What would have been the right way to address this? I sent the apology email quickly because I didn't know what to do.