[Imap][Mail][.net] Attachment constructor: "Invalid Character error " with parameter 'name'

Hello,
I have an issue when instantiating new Attachment

   using (Stream str = new MemoryStream(imap.MessageB))
      {
        attachment = new Attachment();
        attachment.SetContent(str, filename);
      }

with filename="Invoice17/12/9813".
I have many received Gmail mails with attachment that have the '/' inside the filename with no issue. But using Rebex I have "Invalid character at position.." (parameter name).
I tried to change MimeOption to allow everything but still got the error. Any solution?
Thank you