You have configured Outlook to handle your Gmail account as a POP3, yet your sending/receiving test is failing.
1. First, make sure you have correctly enabled POP in Gmail:
a. Sign in to Gmail.
b. Select Settings from the gear in the top right
c. Click Forwarding and POP/IMAP and select Enable POP for all mail or Enable POP for mail that arrives from now on. After saving you it may look like the setting hasn't been saved because the box goes unchecked again but you should see next to the POP Download a status similar to this: "Status: POP is enabled";
2. Assuming all above is done, open Outlook, click Tools menu, select Accounts or Account Settings and on the E-mail tab, click New. Select POP3, IMAP, click next. Use the following:
Incoming Mail (POP3) Server - requires SSL: pop.gmail.com
Use SSL: Yes
Outgoing Mail (SMTP) Server - requires TLS or SSL: smtp.gmail.com
Use Authentication: Yes
Port for TLS/STARTTLS: 587
Port for SSL: 465
Server timeouts: Greater than 1 minute, Google recommends 5
Full Name or Display Name: [your name]
Account Name or User Name: your full email address (include @gmail.com or @your_domain.com)
Email Address: your email address (firstname.lastname@example.org or username@your_domain.com)
Password: your Gmail password
Now if you checked all these steps and you are sure the e-mail and password are correct since you are able to log in to your Gmail account with the same credentials without problem, but your sending/receiving test fails, here is the solution:
1. Go to your Google Account - My Account.
2. Click on Sign in & Security.
3. Scroll down and turn ON "Allow less secure apps";.
That’s it. We hope this tutorial saved you time. Don’t forget to subscribe for more tips & tricks!
Source: Google Knowledgebase