getMessage method
Returns the message body by UID (filename without extension)
Implementation
@override
Future<String?> getMessage(String mailbox, String uid) async {
final mailboxDir = resolveMailboxDirectory(mailbox);
if (!(await mailboxDir.exists())) return null;
final file = File(path.join(mailboxDir.path, '$uid.eml'));
if (!(await file.exists())) return null;
return file.readAsString();
}