Mailspring

Mailspring is a mail client application that lets you connect to multiple e-mail accounts. Using a single inbox for all e-mail accounts helps you get more done in less time.

Mailspring supports every major mail provider—Gmail, iCloud, Office 365, Outlook.com, Yahoo!, and IMAP/SMTP—so you have a single, streamlined command center for all your messages.

Mailspring lets you create unlimited custom signatures and use them across all the accounts in your unified inbox.