If you try to sync some files to an ftp mount you might get the Operation not supported (95) error.

 # cd /media/ftpmountpoint/
 # rsync -rav /tmp/somedata .
 rsync: mkstemp "/media/ftpmountpoint/.somefile.ext.FbGJUT" failed: Operation not supported (95)
 sent 526833 bytes  received 119 bytes  1053904.00 bytes/sec
 total size is 526334  speedup is 1.00
 rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1060) [sender=3.0.7]

Solution:

# mkdir -p /var/tmp/rsync
# rsync -rav --temp-dir=/var/tmp/rsync /tmp/somedata .

Explanation:
The ftp system does not allow creation of temp files. Instead you can direct rsync to create those files locally.

Mouting ftp sites can be done this way:

curlftpfs my.host.tdl /media/ftpmountpoint -o user=username:password