Hello,
I would like to place a "subscribe" link in my email signature, I added this:

/index.php?option=com_acymailing&ctrl=sub&task=optin&hiddenlists=1&user[email]={subtag:email|urlencode}

This redirects me to my site but tells me that the email address is not valid.
An idea ?
Thank you

You have to find a way to replace {subtag:email|urlencode} part with the real email address

Since email clients don't support any kind of programming language, you won't be able to do that

The only option is to add a link to your email signature which would take someone to the page with the subscription form

ok, thank you very much

Not a problem at all