Ever tried to get delisted from Office365 ban list, for whatever reason you might try to get (new IP for a server that was abused in the past or else, you won’t know since they wont tell – and it even looks like they probably dont even really know)?
It is a funny process, because it involves receive a mail from their servers, a mail that will probably be flagged as spam, with clues so big that it might be blocked at SMTP time.
With Exim4, you’ll probably get in the log something like:
2019-08-20 22:18:09 1i0Aa1-0004Hu-8h H=mail-eopbgr740042.outbound.protection.outlook.com (NAM01-BN3-obe.outbound.protection.outlook.com) [188.8.131.52] X=TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256 CV=no F=<firstname.lastname@example.org> rejected after DATA: maximum allowed line length is 998 octets, got 3172
Long story short (this length test is not welcomed by all users), add /etc/exim4/conf.d/main/00_localoptions add
and then restart the server.
Try delisting and check your spam folder. You should get now the relevant mail. Whatever we think about the lenght limit test of Exim4 (based on RCF, isn’t it?), you still end up with a mail sent by Office365 like this:
X-Spam-Flag: YES X-Spam-Level: ***** X-Spam-Status: Yes, score=5.2 required=3.4 tests=BASE64_LENGTH_79_INF, HTML_IMAGE_ONLY_08,HTML_MESSAGE,MIME_HTML_ONLY,MIME_HTML_ONLY_MULTI, MPART_ALT_DIFF,SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=3.4.2 X-Spam-Report: * -0.0 SPF_PASS SPF: sender matches SPF record * -0.0 SPF_HELO_PASS SPF: HELO matches SPF record * 0.7 MIME_HTML_ONLY BODY: Message only has text/html MIME parts * 0.7 MPART_ALT_DIFF BODY: HTML and text parts are different * 0.0 HTML_MESSAGE BODY: HTML included in message * 1.8 HTML_IMAGE_ONLY_08 BODY: HTML: images with 400-800 bytes of * words * 2.0 BASE64_LENGTH_79_INF BODY: base64 encoded email part uses line * length greater than 79 characters * 0.0 MIME_HTML_ONLY_MULTI Multipart message only has text/html MIME * parts
Considering the context, it screams incompetence.