One of the elements of defamation is publication. An agent of a person is not the person. From an old Prosser on Torts:
Calling a person unethical and corrupt, while an opinion, assumes actual facts. While it would certainly be argued that the OP was only expressing an opinion, unless it did not refer to anything else more specific, it would be considered defamatory and not mere opinion. If it was simply saying "unethical and corrupt" (As in "corporation is a stinkyhead".) then the corp would really, really not care and there would be nothing even resembling damages.There may be publication to any third person. It may be made to a member of the plaintiff's family, including his wife, or to the plaintiff's agent or employee. It may be made to the defendant's own agent, employee or officer, even where the defendant is a corporation. The dictation of defamatory matter to a stenographer generally is regarded as sufficient publication, althoug it may be privileged. A few courts, with a tendency to confuse the question of publication with that of privilege, have held that it is not, regarding dictation as an indespensable method in modern business transactions, and therefore merely equivalent to the defendant's own writing.
A good argument for the OP might be the corporation is "public" by its nature and they would need to prove actual malice under New York Times v. Sullivan to win. That would be hard if the OP was simply writing to the corporation on what he felt to be a matter of public concern.

