| Price from | Free | Free |
| Free plan | Yes | Yes |
| Free trial | — | — |
| Open source | — | — |
| Web | Yes | — |
| macOS | Yes | — |
| Linux | Yes | Yes |
| API | Yes | — |
| Self-hosted | Yes | Yes |
| Plans published | 1 | 1 |
| Spec score · Mail Server Software | 8.3 (#4) | 7.8 (#21) |
| Purpose | Citadel is an email, collaboration, groupware, and content-management system. | qmail is a secure, reliable, efficient message transfer agent for typical Internet-connected UNIX hosts. |
| Support | Support is provided through an active user and developer community, an online knowledge base, a support forum, and a mailing list. | The maker directs users to the qmail mailing list and says commercial support is available through qmail.org. |