This error might mean several things:
- The server is busy.
- The connection was interrupted because the IP is not on the RCPT domain whitelist.
- Unacceptable e-mails have been sent from your IP address with unusual frequency. Mail sent from your IP address has been temporary blocked to protect users from spam.
- Temporary system error
- The connection was interrupted because the RCPT domain required that TLS be used
- The connection was interrupted
Ways of solving these
The solution to this error may differ depending on the additional communication from the server that you can find in the log. The following may help:
- Try to connect later. This helps if graylisting is the reason;
- Check if your IP address is present in the RBL databases or other whitelists and blacklists. If it is, then search for ways of removing it;
- It’s quite possible that this error arises along with a 421 SMTP error or greylisting when an SMTP server is being used too heavily. It often comes up when using free SMTP servers, which limit the number of e-mails that can be sent through them each day. This should be resolved by using specialized SMTP servers (such as SendPulse SMTP Server).