Symptom: The messages that you send out come back as a bounce message with the following error:

Cause: SMTP Authentication is not enabled in your e-mail client

Solution: Enable SMTP Authentication in your e-mail client

Symptom: When sending out a message, you receive the following error:

Cause: You have sent more than 300 e-mails during the day thus reaching your daily send limit.

Solution: If the usage is legitimate and you need your limit increased, contact Support. If you did not send that many e-mails, immediately run a full virus scan of your computer, change your email address password or contact your administrator.

In the user control panel, option Track Deliveries, you are able to see how many messages the account has sent out in the last 24h.

Symptom: When sending messages out, the server repeatedly asks for your username & password, despite having entered the right ones

Cause: If you are sending out using the regular port 25, it is possible that your local Internet Provider is intercepting SMTP communications on port 25 and forcing you to use their own SMTP servers. Your SMTP credentials would thus fail against their servers

Solution: Change your outgoing SMTP server port to another value. You can use ports 26 or 587 for regular/TLS server or port 465 for SSL service.

Symptom: When sending out a message, you receive the following error:Sorry, you are blacklisted on this server
Cause: Usually happens when you have your own account in its own blacklist and try to send a message to from/to yourself or when Outlook does its account test, during account setup.

Solution: Access your account’s control panel, Filtering > Blacklist, do a Control F and search for your own e-mail address, then click on the minus sign (-) next to it, to remove it.

Symptom: When someone sends you an email, they get the error below:
552 cuda_nsu sorry, you are blacklisted on this server
Cause: You changed default domain or user filtering settings inside the Filtering option. Instead of using the default Accept e-mails from Everyone, such option was changed to Accept e-mails from whitelist and address book only, which means account will receive e-mails only from addresses in the whitelist or contacts and all other e-mails will be rejected.

Solution: Change the mentioned option to Accept e-mails from Everyone. Verify both on admin (domain level) and user control panels, by clicking on Filtering, in the top menu.