According to the RFC Internet Message Format memo (RFC5322 Section 2.1.1), a line of characters in your email MUST not exceed 998 characters, but SHOULD not be more than 78 characters (excluding the CRLF / line break). If your emails do contain more than these limitations, you will receive bounces and error codes such as:

550 Maximum line length exceeded

550 Message is not RFC5322 2.1.1 compliant

550 Line too long

If you are receiving any of the following error codes, it is likely that a line of your email has not been finished with a CRLF or Carriage Return Line Feed:

451 See http://pobox.com/~djb/docs/smtplf.html

451 stray newline detected. See http://cr.yp.to/docs/smtplf.html

550 5.6.0 Lone CR or LF in headers (see RFC2822 section 2.3)

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