Respources restricted

[TerrificT1953]

I use Outlook for Mac 2021. I am trying to send an email to 20 recipients. I get an error message 5.5.3 Resources restricted from 89.149.26.73 - try again later; then either (AC,6) or AA,6). I've tried sending it to 10 at a time, but get the same error message. I've sent emails to these same 20 recipients before without difficulty. Can someone please help me. I am NOT very computer literate, so please take baby steps if you reply to me.
Thanks so much!