Hi Ferguson2
Yes, knowing the TT bounceback code usually tells me what's happening and in this case tells me that the fail to deliver is because of a temporary stop on accepting mail messages from a particular Gmail outgoing server because of a flood of spam being received.
Usually the message "Outgoing emails identified as spam" accompanies the TT406 code. This does not mean your daughter's mail messages have been identified as spam but simply that the same outgoing mail sender as she is using has been detected by TalkTalk's inbound mail servers as sending spam to the TalkTalk network. It's a blunt message really intended for the Gmail / Google Mail admins to tell them that they need to stop Gmail spammers because enough is enough.
Gmail admins will do two things. Stop the spammers and change the sending IP address for the 'spammy' outgoing server.
Clearly, your son's email got through and my test email got through so not all Gmail outgoing servers are affected but it's now down to how fast Gmail admins can stop the new wave of spam targeting TalkTalk Mail addresses. Last time this happened, some weeks ago, it was mainly tiscali.co.uk addresses being targeted. A combination of reducing the Gmail spam and new spam filters at TalkTalk did eventually make things better.
Your original question was how to unblock (when a TT406 bounceback happens to the potential incoming email). There is no way to unblock or whitelist. As a workaround, your daughter could try sending a different way. If using Gmail webmail doesn't work try using the Gmail app or other email software to see if that gets a new outgoing mail server. Or use a non Gmail address whilst Gmail sort out the spam that's making normal use of Gmail a problem for us all.
