Exim limiting number of emails and cc

The error you got is based on RFC 2822 / RFC 5322 (Internet Message Format):

2.1.1. Line Length Limits

There are two limits that this specification places on the number of characters in a line. Each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters, excluding the CRLF.

The 998 character limit is due to limitations in many implementations that send, receive, or store IMF messages which simply cannot handle more than 998 characters on a line.

2.2.3. Long Header Fields

Each header field is logically a single line of characters comprising the field name, the colon, and the field body. For convenience however, and to deal with the 998/78 character limitations per line, the field body portion of a header field can be split into a multiple-line representation; this is called "folding". The general rule is that wherever this specification allows for folding white space (not simply WSP characters), a CRLF may be inserted before any WSP.

As every decent email client probably conforms to this standard you probably won't reject anything but SPAM if you don't even consider this as a problem and try to solve it.

If you have a long list of addresses in CC: field, your email client should add line breaks before the 988 character line length limit. As the mean email address length is 23 characters, that would be approximately 43 addresses per line. If you need to send an email to this many recipients, please consider using BCC: field (or a mailing list software with opt-out possibility) instead.

Apparently exim has set this limit. This link describes how to modify the

exim4.conf.template in order to avoid messages being rejected.

It's just the case to add IGNORE_SMTP_LINE_LENGTH_LIMIT=1 to the template and regenerate the config file with update-exim4.conf