| Price from | Free | Free |
| Free plan | Yes | Yes |
| Free trial | — | — |
| Open source | — | — |
| Web | Yes | — |
| macOS | Yes | Yes |
| Linux | Yes | Yes |
| API | Yes | — |
| Self-hosted | Yes | Yes |
| Plans published | 1 | 1 |
| Spec score · Mail Server Software | 8.3 (#4) | 7.4 (#28) |
| Purpose | Citadel is an email, collaboration, groupware, and content-management system. | OpenSMTPD implements server-side SMTP and lets machines exchange email with systems that speak SMTP. |
| Support | Support is provided through an active user and developer community, an online knowledge base, a support forum, and a mailing list. | The project is volunteer-developed and is not backed by a commercial entity. |