What does a mailing look like when I have the voting codes sent by Onlinestemtool.nl to the voters? 
The mailing looks clear and user-friendly. See an example here. This states the date and time, but also for which ballot they can vote. In addition, a step-by-step explanation is given of how voters can cast a vote, including a link to the voting website. Of course you can also pass on changes yourself or add extra information in this email. Consider, for example, the link of an online meeting or additional information about the vote.