You can use hotkeys to add single-line underline for the selected text with hotkeys in the message body.

1. In the message body, select the text you will add underline for.

2. Press Ctrl + U keys together.

Now the selected text is marked with a single-line underline.

when you are composing, replying, or forwarding an email, you can also apply the Underline feature to add a single-line underline for the selected text in the message body.

1. In the message body, select the text you will add underline for.

2. Go to the Font group on the Format Text tab, and click the Underline button .

Tips: You can also get the Underline button  in the Basic Text group on the Message tab.

Now the selected text is marked with a single-line underline.

If you need to mark the selected text in the message body with different style of underline, you can open the Font dialog and get it done.

1. In the message body, select the text you will add underline for.

2. Go to the Font group on the Format Text tab, and click the anchor  at the bottom-right corner to open the Font dialog.
Tips: You can also press Ctrl + D keys together to open the Font dialog.

3. In the Font dialog, enable the Font tab, and do as follows:
(1) Select an underline style from the Underline style drop-down list;
(2) If you need to color the underline, please select a color from the Underline color drop-down list.
(3) Click the OK button.

Now the selected text is marked with custom style of underline.

You can easily clear the underline formatting as follows:
(1) Select the text with underline;
(2) Click the Underline button in the Font group on the Format Text tab (if the text is marked with a custom style of underline, you need to click the Underline button twice).