Apple Mail Sends Messages from Wrong E-mail Address

Recently, when I send messages from my iMac or MacBook Air, the message I compose defaults to one of my other email accounts instead of my iCloud account. When I go to the ‘from’ field to try to change the address to iCloud, my iCloud addresses (I have 3) have disappeared from the drop down list. When I go to settings/ composing, the ‘send from’ default list is missing my I loud accounts. This happens occasionally and eventually corrects itself, however if you’re expecting mail to ‘just work’, you send an email from an address that the recipient’s email does not recognize and places it in their junk mail. This is serious especially for emails involving personal or important information. Have made 2 calls to Apple on this and they have no solution so far. Are others experiencing this? Is it the latest iOS update perhaps? Thanks in advance for your replies!