According to their own accounts, they have not taken any venture capital or loans. And they say they're donating to NGOs on a regular basis. I could also imagine that users' deposits are invested. (They use a pre-paid model, and e.g. I always send 25 or 50 euros at a time when recharging my Posteo account. An average of maybe 5 euros times 50,000 users is a quarter million in free cash.)
Source: https://posteo.de/en/site/sustainability