If You Have A GRU.Net Account But Sometimes Use A Different Internet Service Provider Other Than GRU.Net And Need To Send Mail, Use The New Message Submission Port 587:

In order to help restrict "spam," "worms," etc., many Internet Service Providers (ISPs) including GRU.Net  require that all outbound e-mail on the "default" SMTP port 25 use only our "official" GRU.Net server(s). However, it also severely complicates life for users of mobile/laptop computers, who frequently connect to different networks, requiring changes to their e-mail program configuration with each change of location.

Fortunately, GRU.Net now supports an alternative means of e-mail submission; the Message Submission Port, port number 587. It is specifically designed to be the entry point for new messages, and requires "secure authentication" using TLS. The Message Submission port does not accept messages from unauthenticated sources, helping to protect networks and users from malicious e-mail.
After configuring your e-mail program to send outbound mail via the Message Submission port, you will probably find that you are prompted for your username and password the first time you send a message during each e-mail session. Generally, after you authenticate for the first outbound transmission, your system will remember your credentials as long as you don't exit/quit the program.
The Message Submission Port Essential Facts: 

GRU.Net Outgoing E-mail Host	smtp.gru.net
Message Submission Port	587
Authentication Required?	Yes, Username & Password
Connection Encryption	TLS (note; SSL is not TLS)

For Further Assistance

If you need additional assistance in configuring your e-mail program, please contact the Support Desk at 352-334-3100 or support@gru.net.