How to omit/alter the sigend-off line in a patch set?

Hello,

  I am creating a patch set in this way:
I make a change in one file, than commit it. (git commit file1)
Then I make a change in a second file, than commit it. (git commit file1)
and so on.

Then I create a patch set thus:

git-format-patch -M -s -n -o patchdir origin

And a number of patch files are created, according to the number of commits I had made.

Now, in each created patch file there is a line saying:
Signed-off-by: root <root@hostName.(none)>

Which is caused as I am doing this patches as root, and the machine name is hostName (and the domain name is none).

Now, is there a way to cause git-format-patch to omit this
Signed-off-by: root <root@hostName.(none)> line ?
Or, altetnatively, is there a way to cause the git-format-patch to
generate a differnet
signed off by line (like signed off by ianbrn@gmail.com)?

I had tried using the --signoff option for git-format-patch , in these two ways, but got an error:

git-format-patch --signoff=ianbrn@gmail.com -M -n -o patchdir origin fatal: unrecognized argument: --signoff=ianbrn@gmail.com

git-format-patch --signoff ianbrn@gmail.com -M -n -o patchdir origin fatal: ambiguous argument 'ianbrn@gmail.com': unknown revision or path not in the working tree. Use '--' to separate paths from revisions

I also tried to use the -a flag for commit git-commit -a ianbrn@gmail.com but again it did not help either.

Any ideas?

Any help will be appreciated,

Regards, Ian