Message text containing more than 1000 characters in a single line are modified by MailGate.

This is an expected behavior of MailGate and is normal according to RFC 821 (2821) & 822.

According to RFC 821:

text line

              The maximum total length of a text line including the
              <CRLF> is 1000 characters (but not counting the leading
              dot duplicated for transparency).

According to RFC 2821:

text line
     The maximum total length of a text line including the <CRLF> is
     1000 characters (not counting the leading dot duplicated for
     transparency).  This number may be increased by the use of SMTP
     Service Extensions.