Link does not show up in email receipt

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In the website receipt, I have set up a link so that customers
can easily sign up for my client's newsletter.

This link works well on the webpage receipt, however, it
does not show up as a link in the email. Just the word
"Subscribe"

However, two other links in the email receipt which go to
my client's website work fine.

How can I make the email link function properly?

Thanks,

Marje