You have configured Outlook to handle your Gmail account as a POP3, yet your sending/receiving test is failing.
First, let's make sure you have the correct settings:
1. Verify if 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. Is POP correctly enabled? Great! Now let's verify the Outlook settings:
Incoming Mail (POP3) Server - requires SSL:
Outgoing Mail (SMTP) Server - requires TLS or SSL:
Port for TLS/STARTTLS:
Port for SSL:
Greater than 1 minute, Google recommends 5
Full Name or Display Name:
Account Name or User Name:
your full email address (include @gmail.com or @your_domain.com)
your email address (firstname.lastname@example.org or username@your_domain.com)
your Gmail password
So, your POP is correctly enabled, your Outlook settings are correct, your e-mail and password are correct, yet your sending/receiving test still fails. This is the solution!
1. Go to your Google Account - My Account.
2. Click on "Sign in & Security".
3. Scroll down to "Apps with account access" and turn "Allow less secure apps" ON.
That’s it. We hope this tutorial saved you time. Please, leave you feedback below!
Source: Google Knowledgebase