There is a report on bugzilla about this: https://bugzilla.mozilla.org/show_bug.cgi?id=300937 However, looking at that RFC, there are several factors that might generate this error, and it isn't at all clear from the error message you've shown us quite which part of the message has offended the smtp server. https://tools.ietf.org/html/rfc2821#section-4.5.3.1 I wouldn't be surprised that this happens in association with html. An html encoded document could well be presented as a single long line of text, since its own logical line ends are encoded into it and don't bear any relationship to the "physical" line ends that the smtp server is concerned with. As an example, the two paragraphs above could be encoded as one long line: https://tools.ietf.org/html/rfc2821#section&#8209;4.5.3.1&nbsp;&lt;br&gt;I&nbsp;wouldn't&nbsp;be&nbsp;surprised&nbsp;that&nbsp;this&nbsp;happens&nbsp;in&nbsp;association&nbsp;with&nbsp;html.&nbsp;An&nbsp;html&nbsp;encoded&nbsp;document&nbsp;could&nbsp;well&nbsp;be&nbsp;presented&nbsp;as&nbsp;a&nbsp;single&nbsp;long&nbsp;line&nbsp;of&nbsp;text,&nbsp;since&nbsp;its&nbsp;own&nbsp;logical&nbsp;line&nbsp;ends&nbsp;are&nbsp;encoded&nbsp;into&nbsp;it&nbsp;and&nbsp;don't&nbsp;bear&nbsp;any&nbsp;relationship&nbsp;to&nbsp;the&nbsp;"physical"&nbsp;line&nbsp;ends&nbsp;that&nbsp;the&nbsp;smtp&nbsp;server&nbsp;is&nbsp;concerned&nbsp;with&#46;&lt;br&gt;<br> But I'm not sure if that's relevant. We can't tell which kind of line has given rise to the error message. :-( It might be the subject or the To: line and nothing to do with the message body. Can you send these messages via a different smtp server to test if this is an idiosyncrasy of the particular smtp server you have been using?