If you mean a link in the Safari web browser, there are several ways to get back to the Mail app:
While in Safari, you can double press the Home button (or use the four-finger swipe up gesture) to reveal the multi-tasking bar. The Mail app should be the first icon on the left; press it to open the app.
Or, you can use the four/five-finger multi-gesture to swipe to the left (revealing the Mail app from the right).
Or, you can click the Home button once to close Safari, then find/click on the Mail app.
Hope this helps.
Marilyn