Can parties mobilize young people by sending letters to first-time voters? Parties are in a constant competition for new voters. First-time voters are a particularly attractive target group as their first vote choice has a long-lasting effect on partisanship. A common and costly instrument that many parties use are mailed letters, in which they directly appeal to first-time voters. However, it is unclear whether such letters really have the potential to mobilize young citizens. We address this question by causally identifying the effect of party letters on turnout and vote intention of first-time voters. We conducted a large-scale field experiment in collaboration with the German Green party in the context of the European Parliament Election in 2024. Overall, 37,000 letters were sent to first-time voters in the city of Berlin. We randomized which first-time voters receive a letter based on the zip code area in which they reside. We find no effects on turnout or the party’s vote share in the treated localities. These findings are complemented with an individual-level analysis based on a post-election survey that we fielded among first-time voters. The study makes an important contribution to our understanding of political mobilization, political campaigning more generally and about political mobilization of young voters in particular.