There are several possible causes. Please read the article

Symptoms

An attempt to send a test E-mail notification fails with the following error message:

Mail server error

Cause

There are several possible reasons:

• A Firewall or your provider blocks the outgoing SMTP connection;
• The connection is lost while sending an E-mail;
• Not all of the E-mail settings were entered correctly

Solution

1. Make sure your Firewall does not block your outgoing network connections via the selected SMTP server port. Make sure your Firewall does not block the TrueImage.exe process;
2. Check that your network provider does not block your outgoing network connections via the selected port. You can use telnet to check this. See Using telnet to Test Open Ports;
3. Try using other ports (using encryption) or other SMTP servers;
4. Check your network connection and network access to the SMTP server. For example, do this with the ping command;
5. Make sure all the E-mail settings are correct. Check Gmail settings here and Yahoo/Hotmail/Live/Outlook settings here.

T-online Mail
Outgoing mail server: securesmtp.t-online.de
Port: 587
Encryption: TLS
Username: your T-online e-mail address
Password: your T-online e-mail password

Yandex
Outgoing mail server: smtp.yandex.ru
Port: 465
Encryption: SSL
Username: your Yandex e-mail address
Password: your Yandex e-mail password