One of my corpmates said that “back in the day” EveMon was able to get corporate mails for notification of POS/Structures being attacked. I have looked at the current API and can find no method that returns corporate mail.
So you can use the character’s mail endpoint to also get corp mails, as its the same thing as a personal mail but with a diff label. However i think you’d moreso be interested in EVE Swagger Interface, which includes corporation notifications such as WarDeclared or OwnershipTransfered etc.
Labels appear to be limited to mail sent to me. What I’d like to get are the messages that show on the Communications / Structures tab. Specific example is POCO under attack message. Do you know if these are available?