My memory isn't the best, but I'm fairly sure the duplicate problem posted here a few days ago was also from an Optimum user, but I can't be positive. This is a server issue, and the technical explanation is over my head, but email clients cannot cause this problem. I encourage you to contact them to see what assistance they can provide. If I locate the prior post, I'll let you know. And maybe someone on this forum with more knowledge on this error may see this and comment.

YES! Here it is: https://support.mozilla.org/en-US/questions/1386455 same provider, Optimum. I can't fix this, and I hope when you talk to them, they may take this seriously.