Password Reset After Sending Specific Group Email

I have 4 separate groups set up to send personal emails. Three of the groups work just fine, but one group (no matter which order I send it) always causes a password reset. This might be over a week or two but always every two months when I send regular reminder emails. Why is this happening? It’s a pain to reset my password so frequently on all three Apple devices. I have called C/S until I’m blue in the face but no one has an answer.