how to underscore a email address on macbook pro

In which application? Typically the underscore character _ is used instead of a space in email addresses. Many applications will automatically convert valid looking email addresses such as name@domain into a link, applying a colour and underscoring to all of the text to make the link stand out. The forum software omits the underscoring but I can use the editing tools here to include it: name@domain. If you want to highlight part of an address then it may be better to use bold.

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In Mail, select the email address and go to the menu Format > Make Plain Text.

Then you can underscore what you want.

Post back with the application want to do this with if it is not Mail.

MtD