António, can you verify in the matching sent items of this email message that the DKIM-Signature is all on one line. It is possible to split it into multiple lines as line as each line is preceded with a whitespace character (typically a space or a tab). Viewing it with in a notepad or other text editor with a fixed-width character font would help determine that. If this is not the case, then it would fail DKIM. If you have questions you can also send me a copy of the headers, and I can verify for you.