This article is from the ZTerm FAQ, by Les Jones email@example.com with numerous contributions by others.
Joel Schulman wrote to say that he couldn't download binaries until
he began using the -e modifier, as in
sz -e filename
Several readers pointed out that the -e modifier escapes control
characters such as control-s and control-q. These are the XON/XOFF
characters used in software flow control. If UNIX sees the XOFF
character, it stops the file transfer. The -e option tells the UNIX
host to ignore these characters.