For sending email to all contacts in Outlook at once, please do as follows.

Part 1: Create a distribution list with all contacts inside

1. Get into the Contacts view by clicking Contacts or People in the Navigation Pane.

2. Click Home > New Contact Group to create a new contact group.

3. Then a dialog box pops up, please give a name to this new distribution list by typing words in the Name textbox. See below screenshot:

4. Click Add Members, and select From Outlook Contacts from the drop down list. See above screenshot:

5. In the Select Members dialog box, please select all contacts and then click Members -> button. Then click OK button.

Notes:
(1). For quickly selecting all contacts, please select the first contact and the last one with holding the Shift key simultaneously.

(2). If your contacts are populated in several contact folders, you can choose other contact folder from the Address Book drop down list and go on adding them to the Members box.

6. Then click Save & Close button in the Contact Group dialog box.

Part 2: Sending email to this created distribution list

Now you can send email to the distribution list you have created above.

1. Go back to the Mail view by clicking Mail in the Navigation Pane.

2. Click Home > New E-mail to create a new email message.

3. In the Message window, please click To button. See below screenshot:

4. In the Select Names: Contacts dialog box, select the contact group with all contacts you have created just now and then click To button. And finally click OK button as above screenshot shown.
Now the specified contact group with all contacts has been added into the To box as below screenshot shown.

5. Compose your email and send it.

One click to remove all duplicate recipients from the To/Cc/Bcc fields in an email in Outlook

Sometimes, you may add many recipients when composing an email in Outlook, and duplicate recipients are likely to come out in the To/Cc/Bcc fields. With Kutools for Outlook's Duplicate Names feature, you can quickly remove any duplicate recipients from he To filed, Cc field, and Bcc fields in an composing email.