Make the posteo test pass

src/dc_receive_imf.rs

@@ -2368,7 +2368,7 @@ mod tests {
             "hanerthaertidiuea@gmx.de",
             "04422840-f884-3e37-5778-8192fe22d8e1@posteo.de",
             include_bytes!("../test-data/message/posteo_ndn.eml"),
-            "Delivery Status Notification (Failure) – ** Die Adresse wurde nicht gefunden **\n\nIhre Nachricht wurde nicht an assidhfaaspocwaeofi@gmail.com zugestellt, weil die Adresse nicht gefunden wurde oder keine E-Mails empfangen kann.\n\nHier erfahren Sie mehr: https://support.google.com/mail/?p=NoSuchUser\n\nAntwort:\n\n550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient\'s email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser i18sor6261697wrs.38 - gsmtp",
+            "Undelivered Mail Returned to Sender – This is the mail system at host mout01.posteo.de.\n\nI\'m sorry to have to inform you that your message could not\nbe delivered to one or more recipients. It\'s attached below.\n\nFor further assistance, please send mail to postmaster.\n\nIf you do so, please include this problem report. You can\ndelete your own text from the attached returned message.\n\n                   The mail system\n\n<hanerthaertidiuea@gmx.de>: host mx01.emig.gmx.net[212.227.17.5] said: 550\n    Requested action not taken: mailbox unavailable (in reply to RCPT TO\n    command)",
         )
         .await;
     }

src/mimeparser.rs

@@ -894,7 +894,7 @@ impl MimeMessage {
         if let Some(original_msg) = report
             .subparts
             .iter()
-            .find(|p| p.ctype.mimetype == "message/rfc822")
+            .find(|p| p.ctype.mimetype.contains("rfc822"))
         {
             let report_body = original_msg.get_body_raw()?;
             let (report_fields, _) = mailparse::parse_headers(&report_body)?;