We allow long lines in email messages.
RFC5322 for 'Internet Message Format' says that each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters.
However, there are many email systems (eg recent Outlook versions) that will not wrap long lines properly. As a result, rather then us rejecting such messages, our incoming and outgoing email servers will allow long lines.
Since September 2023, when we are given an email with lines over 998 characters we will add a header to explain this. Example:
X-AA-RFC5322: Message exceeds RFC 5322 line Length Limits, but allowing it. (20479/998)
The 20479 in this example is the length of the longest line in the message.
(The reason for us adding a header at this point in time is that our email server software (Exim )now enforces the RFC 5322 line length limit, and we have had to change our configuration so that it won't enforce.)