iCloud custom domain email problems

a) my iCloud email is USER@icloud.com

b) I have 3 emails associated with a custom Domain in iCloud: Dom1@mydom.com, Dom2@mydom.com, Dom3@mydom.com

c) these WORK. I can send email to them, they arrive as expected. I can send out from them. they work.

d) if I send an email to USER@icloud.com, Dom1@mydom.com, Dom2@mydom.com, Dom3@mydom.com (my main iCloud email and the 3 domain emails) in ONE message invariably ONLY ONE OF THEM ARRIVES. I get no bounce notice for the ones that get lost.

e) if I edit the original message, deleting 3 of the other addresses, leaving 1 that failed to arrive previously, but literally as in the original (I don't retype it), and send it, the email will arrive successfully.

f) so, if I address an email to all 4, 1 arrives. If I address individually each will arrive ???

g) does anyone understand this ? confirm it ?

thx