In January 2016 the US Office of Foreign Assets Control adopted an important exemption for US companies in the context of the liberalisation of sanctions with Iran.
General Licence H is designed to stop US owned subsidiaries established in the EU (or other countries outside the USA) from having to comply with stricter US Iran sanctions rules thereby being placed at a local competitive disadvantage.
In summary the measure allows such subsidiaries to work independently of their US parent and sell and export to Iran subject to compliance with local laws.
Clearly companies will need to be certain that General Licence H applies to their EU activities (cf subsidiary vs branch). Then the issue becomes one of EU compliance – an issue which most EU subs have not previously had to deal with.
EU sanctions with Iran have been liberalised - but the compliance burden in many senses still remains high. Compliance (but not commerce) was in retrospect easy when all transactions were prohibited. Now that transactions are permitted subject to screening for names and end-use and potentially subject to national licensing procedures, the administrative burden becomes high.
The message is that trading with Iran requires careful and consistent attention.
Resources: https://www.treasury.gov/resource-center/sanctions/Programs/Documents/iran_glh.pdf