Identical email sent to my children

[#1]

Sent together using "To" option , "Copy" and "BCC" not used. One recipient is with ANOther ISP, and the other two are Plusnetters like me.
One Plusnetter and the ANOther client  both received their mail, but the other one did not, and also a later attempt by using "Foward" also failed. In neither instance did I receive a failure notice.
Suggestions, please, why one Plusnet customer and not the other should receive identical mail from the sender, also with Plusnet.
I'm puzzled!

[#2]

Was it the last one of the three that lost out? Just wondering if there's an unwanted/illegal character at the end of that address?

[#3]

No, the middle one, the first being the one to the non-Plusnet address. i examined the address and it conforms to the return path in an email sent me from it, Also I replied to another email sent me and my reply got back too.
So it was one that did not get through for noe logical reason. The same happened to me a few years back with mail incoming from another unrelated PN address. The only way I could be sure to get his mail was whitelisting.

[#4]

I presume the lost message hasn't been put into the Spam folder?
Rather clutching at straws but is 'discard obvious spam' set in Manage My Mail?
(Concerning the previous problem you had, do you have 'discard obvious spam' set?)

[#5]

For my incoming mail, yes. But how my daughter's is set up, I have no idea. She and partner are however  somewhat less clued up than me!:o

[#6]

I think 'discard obvious spam' set to 'On' is the default so probably your daughter has that set too. It's one place the missing message could have disappeared to. I have mine set to 'Off' and suggest making that change - I barely receive any spam even with this setting. You mentioned having to whitelist someone on a previous occasion, I think whitelisting bypasses internal spam checking (but not external checks).
The other place to look is the Spam folder. (I know the other Plusnet addressee received it - but I wouldn't rule out the possibility of different behaviour.)

[#7]

Found out something, that ATM they only can use Plusnet's webmail service, having recently replaced their umpteen year old XP computer. They're waiting on acquiring 7 via a special scheme.