This is a known issue with API Portal versions older than 4.2.8. The internal PSSG server attempts to contact the email server using credentials when there isn't any to provide. This causes the internal PSSG assertion to fail.
Please upgrade to 4.2.8 for a permanent fix.
If you have an older version of API Portal and you need a workaround. Please contact CA Support on how to access the internal PSSG to edit the "PSSG Portal Auth Policy". There is a "Send Email Alert" assertion around line 321 that needs to be modified to disable the user name and password requirement. This is a short term fix because a portal restart will revert these changes.
Please note: This resolution applies to non-SSL SMTP external servers that do not require user name and password.