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Log into your MailUp admin console and look at the browser address field. It will look something like abcd.emailsp.net or a4h4i.s21.it (or alike).
API User Name
Once logged into the MailUp admin console:
1: Go to Settings > Account settings > Developer's corner. On the Developer's corner page, click on Web services. 
2: The WS User Name is your API user name. It starts with the letter "a" followed by your numeric account number.
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For your internet and application security, enter a strong password (6-8 characters long, case-sensitive, include including special characters) in the password field and click on Save.
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Debugging
The debugging option is for mostly for internal purposes. It enables more detailed errors in certain scenarios (e.g. during import). Typically, it should remain unchecked (disabled).