If you have not changed any settings and your mail relay account stop working. First, you need to make sure your mail relay is at good status by login to your DNSExit.com account. After you login, click on Manage under Mail Relay next to the mail relay account name to enter Mail Relay Control Panel. At the control panel, there is a link Verify Your Mail Relay Account to verify that your mail relay is active and working.
If the account is working but your mail relay stop working without changing settings, it is highly possible that you the SMTP port you use to send out emails are blocked by either your ISP or firewall. You can use the DOS telnet command to test out if the port is blocked or not. Most likely, you are using port 25 which is the standard SMTP port to send out emails. Try command
>> telnet relay.dnsexit.com 25
and if the port is not blocked, you should see
Connected to relay.dnsexit.com.
Escape character is '^]'.
220 box20.911domain.com ESMTP Sendmail 8.14.4/8.14.4; Mon, 22 Oct 2018 00:29:18 -0400
If not, then the port is blocked. You can try other ports with command
>> telnet relay.dnsexit.com 25
>> telnet relay.dnsexit.com 26
>> telnet relay.dnsexit.com 940
>> telnet relay.dnsexit.com 8001
>> telnet relay.dnsexit.com 2525
to find a working port and switch to the new port to send emails. Port 80 is usually for web serving and it should never got blocked, you can always use port 80 to send emails.
If you are using MicroSoft exchange server for our mail relay service as smarthost, you can follow this document to change the smarthost sending port: