Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
FreshBooks - The ideal accounting software for small business owners.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
Sage Intacct - Intacct is a cloud-based accounting and financial management system that is designed for the businesses of all levels.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
QuickBooks Online - QuickBooks Online is a Cloud Accounting for small business that can be used on iPad, mobile phone or computer to do invoicing, billing, run reports, or even pay employees online.