There have been issues regarding the widely used Microsoft Outlook, Mozilla Thunderbird, Apple Mail, and etc of late. The issue seems to be the blocked port 25.
Source: UniFi website
Steps to switch from default SMTP port 25 to 587
- Launch Apple Mail
- Drop down the mail menu, and choose preferences
- Select the email account to modify under accounts
- Beneath Outgoing server (SMTP) click on the drop down bar and select "Edit SMTP server list...".
- Click on your desired Outgoing server and then click advanced.
- Select option for custom port and type in 587.
- Click OK
- Restart Apple Mail
Microsot Outlook and Outlook Express
- In Outlook, drop down the Tools menu
- Choose Accounts or Email Accounts
- View or change existing e-mail accounts should be marked
- Click Next
- Select the account and click Change
- Select More Settings
- In Outlook Express, drop down the Tools menu
- Choose Accounts
- Select the account and click Properties
- Click on the Advanced tab
- Change the outgoing mail from port 25 to port 587
- Make sure that the box that says This connections requires a secure connection(SSL) is NOT checked
- Click OK
- Restart Outlook