If you are using Exim to transfer mails generated by internal software you probably often get mails with a really weird looking subject that starts with something like this:

=?utf-8?Q?=5BPersonalverwaltung_=2D_Fehler_=23=31=37=38=5D_...

This problem arises when some system sends an email with more than 76 characters in the subject line (usually people don't do that).

While there is no limit to the length of a multiple-line header field, each line of a header field that contains one or more encoded-word's is limited to 76 characters.

The length restrictions are included both to ease interoperability through internetwork mail gateways, and to impose a limit on the amount of lookahead a header parser must employ (while looking for a final ?= delimiter) before it can decide whether a token is an "encoded-word" or something else.

Excerpt from RFC2047

Fix It

You can remove the length checking by adding the following to the main section of your /etc/exim4/exim4.conf. The main section usually ends with the first begin in that file.

check_rfc2047_length = false

For further reference I recommend the official Exim documentation