Default configuration set with SES IAM user

Hi,
I linked a default configuration set with a verified identity in SES. The configuration set transfers SES data via Fireshose to S3. All works fine when using the 'Send a test email' in SES.
I also created a PHP script, using the same SES verified email address, to send outbound emails. Here too, without specifying the 'X-SES-CONFIGURATION-SET' custom header parameter, the default configuration set is used to transfer the data correctly to S3. This uses a SMTPUser/pwd combination.
When using the same verified email address via an IAM user, the emails are sent out correctly, but the default configuration set doesn't seem to be taken into account. No data gets sent to Firehose (and so neither to S3).
It looks to me like a permission issue, but I tried already different things and nothing seems to fix the issue.
Anyone an idea what I missed?
Thanks,
Nico