For many years sending email via SMTP required no authentication. Especiallywhen you’re connected via the ISP’s provided lines equivalent to dialup or broadband,that was enough to “authenticate” that you simply are authorized to use the ISP’sservers. Many still function that way by default but impose some simplerestrictions similar to requiring that the “from” address be something that theISP controls. Quite often the ISP will lift regulations so long as SMTPauthorization has been successfully used.
Try configuring the account that’snot operating to use authentication either “same as receiving account”, ifthat’s an option, or with specific account/password suggestions.