The OpenSMTPD project is developed by volunteers and is not backed up by a commercial entity.
As a general rule, you can help the project by donating to the OpenBSD foundation whose goal is to fund OpenBSD and related projects, including LibreSSL upon which we rely. The donations make the OpenBSD infrastructure better and lets OpenBSD organize hackathons where we tend to do a LOT more work than usual.
Alternatively, if you wish to make a donation to the OpenSMTPD project itself or to a specific developer, you can use the button below or send a paypal to firstname.lastname@example.org letting us know in the comment where this should go.
Finally, some developers are enrolled in community-funding programs, whereas others may be contracted to work on sponsored development and issue invoices.
Feel free to reach out to the developers through the mailing list, direct e-mail or official IRC channel (#opensmtpd @ freenode) to get more details.