This method will guide you to insert a horizontal line in an email, and then copy it to the specified signature in Outlook.

1. In the Mail view, create a new email with clicking Home > New Email.

2. In the new Message window, put the cursor in the message body, and click Format Text > Table > Horizontal Line. See screenshot:

3. Select the horizontal line, right click and select Copy from the right-clicking menu.

4. Click Insert > Signature > Signatures.

5. In the opening Signatures and Stationery dialog box, please:

(1) In the Select Signature to edit section, select the specified signature you will add a horizontal line;
(2) In the Edit signature section, put the cursor where you will add a horizontal line, and press the Ctrl + V to paste the horizontal line;
(3) Click the OK button.

6. Close the new email without saving.

Auto add text and current datetime/timestamps to subject or signature for Outlook messages

Provided by Kutools for Outlook.

If you are familiar with HTML code, you can add a horizontal line into the signature by modifying its HTML file.

1. Open the folder containing Outlook signatures with pasting below folder path into the Address box of a folder and pressing the Enter key.
Folder path of Outlook signatures: %App Data%\Microsoft\Signatures

2. Right click the specified signature’s HTML file, and select Open with > Notepad from the right-clicking menu. See screenshot:

3. In the opening Notepad, go to the place where you will insert a horizontal line, and type the code of <hr />.

4. Save and close the Notepad.

So far a horizontal line has been inserted into the specified Outlook signature already.

Resize blurry image in signature in Outlook

Open the folder containing Outlook signatures

Import or insert HTML signatures in Outlook

Assign auto signature to every email in Outlook

Align or float images in Outlook signatures