eM Client 10.4.5642, Windows.
Symptom: newsletters from one specific sender open as a completely blank reading pane. Ctrl+A in the pane selects nothing, so no content reaches the renderer at all. The plain text alternative displays normally. Opening the saved .eml file directly gives the same blank result, which rules out the account, the database and the cache.
Cause: the sender’s HTML contains five Outlook conditional comment blocks that are never terminated. They open with instead of <![endif]–>. The document has five openers and zero terminators.
I isolated this with a test set that changes exactly one variable per file:
- Original, quoted-printable, broken terminators: blank
- Terminators repaired, quoted-printable unchanged: renders correctly
- HTML unchanged, re-encoded as base64: blank
- Terminators repaired, re-encoded as base64: renders correctly
- Conditional blocks removed entirely, base64: renders correctly
Files 2 and 3 are the decisive pair: the encoding is not involved, the unterminated conditional comments are.
The message is otherwise valid. Correct MIME structure, valid quoted-printable, valid UTF-8, balanced HTML and CSS. Outlook, Chrome and Firefox all display it without problems, because they either evaluate the condition or treat the blocks as ordinary comments. eM Client appears to handle these blocks the classic Internet Explorer way: searching for <![endif]–> and discarding everything from the onwards when it is missing.
The sender’s template is at fault and I am reporting it to them as well. Even so, malformed comment syntax resulting in an entirely empty message seems worth handling more gracefully, since other clients render it fine.
I can supply all five .eml test files on request.
Regards,
Otto