Mass mailing not concealing recipients

I received an email which the sender tells me was sent as “mass mail” under a Pro licence, but all recipients were shown in the message I received

It is unlikely that they had selected mass mail, as that would have sent an individual message to each recipient.

You could ask the sender to try again, and this time, using the drop down arrow next to Send, select Send as mass mail.

Thanks Gary. They said they had done as you say, but I will get them to try again.