If Zabbix-Server generates e-mails for sending them as action operation, the message body is base64 encoded. Regardless of the length of the message, zabbix generates on big bulk base64 block.
This can cause trouble, because RFC5321 states clear, that a line has to be 1000 bytes or longer, zabbix is not taking care of this.
In our scenario, zabbix creates a mail (say 2500 bytes in size), delivers it to on of our postfix instances. Postfix itself cuts the mail at a point specified with the smtp_line_length_limit parameter. Postfix inserts a <cr><lf><space> per default every 990 bytes.
These mails can not be longer display by Apple Mail, the base64 decoding stops at the first occurence of <space>.
Other clients (Thunderbird, Outlook) seem not be such picky about the format, the message are displayed properly.