http://www1.toutapp.com/tour/tout-integrations
offers native integrations only, not an open API.
In that context no external entity can offer integration since they do not disclose any specification or access.
http://www.yesmail.com/email-marketing-platform/technology/integration-layer/api
It is more flexible than Tout, although not an open API such as SFDC, Marketo, SugarCRM or Google, among many others
.
Acording to their own statement
--- quote
"While most solutions offer canned APIs, which means the customer has to adapt around technological limitations, our API adapts around your needs.".
--- end of quote
That is the strongest indication they develop the API themselves and keep it closed rather than disclosing it users and 3rd parties.
In that context Marketo or any other vendor can not offer that integration since Yesmail is not making their specifications publicly accessible.