What is the correct syntax for a macro with >1 parameter where 2nd etc have defaults but not the 1st?

The document that I am working with has a macro with 1 parameter:

newcommand{emailTo}[1]{The email address that #1 is sent to.}

I have to add another parameter with a default value of email address to remain backwards-compatible. I tried:

newcommand{emailTo}2][][email address]{The #2 that #1 is sent to.}

But I am getting this error:

You can't use `macro parameter character #' in horizontal mode. ...emailToNew}[2][qeee][email address] {The #

documentclass[10pt,a4paper]{article}
usepackage[utf8]{inputenc}

newcommand{emailTo}[1] {The email address that #1 is sent to.}

newcommand{emailToNew}[2][][email address] {The #2 that #1 is sent to.}

begin{document}
    emailTo{notification}
    
    emailToNew{notification}%{email address}
end{document}

Is there any way to stay backwards-compatible and not have to change hundreds of instances where the existing macro is used with 1 parameter?

……

documentclass[10pt,a4paper]{article}

NewDocumentCommand{emailTo}{mO{email address}}{%
  The #2 that #1 is sent to.
}

begin{document}

emailTo{somebody}

emailTo{somebody}[parcel]

end{document}