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

Apple Mail

1. Launch Apple Mail
2. Drop down the mail menu, and choose preferences
3. Select the email account to modify under accounts
4. Beneath Outgoing server (SMTP) click on the drop down bar and select "Edit SMTP server list...".
5. Click on your desired Outgoing server and then click advanced.
6. Select option for custom port and type in 587.
7. Click OK
8. Restart Apple Mail

Microsot Outlook and Outlook Express

1. In Outlook, drop down the Tools menu
2. Choose Accounts or Email Accounts
3. View or change existing e-mail accounts should be marked
4. Click Next
5. Select the account and click Change
6. Select More Settings
7. In Outlook Express, drop down the Tools menu
8. Choose Accounts
9. Select the account and click Properties
10. Click on the Advanced tab
11. Change the outgoing mail from port 25 to port 587
12. Make sure that the box that says This connections requires a secure connection(SSL) is NOT checked
13. Click OK
14. Restart Outlook