How well does each app integrate?
Each app wants to access the OS X address book, which Inky also uses to maintain an inbox of messages from your contacts. If you email yourself a reminder, Inky files it as a note, rather than a message. Clicking on a date in an email in Airmail lets you add it to the Calendar, Reminders or Things. You can also link cloud storage services for hosting attachments rather than sending them within the body of an email, and Evernote for saving messages. Postbox can do both of these too, as well as posting to social networks. Thunderbird also allows you to host attachments on cloud servers, has its own contact book and, like Postbox, can be augmented using plug-ins, so you can bundle in a calendar and use both to read RSS feeds and Usenet groups. Neither Unibox nor Sparrow creates links from dates or identifies notes, however you can use a cloud service to host Sparrow attachments and link it to Facebook to download your contacts’ avatars. Lastly, Unibox shows avatars for contacts with accounts at Gravatar.
Airmail is one of the most flexible options for linking emailed dates to calendars.