What are some alternatives?
When comparing Flexmail Solution and CO-SENDER, you can also consider the following products
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
1and1Mail - 1and1Mail is a free email marketing software to send out mass email campaigns, manage email contacts and design email newsletters for small and medium business.
SendPulse - Maximizing e-mail marketing open rates automatically
shiftmail - Email analytics made simple. shiftmail provides a REST API and web interface for intelligence gathering within email marketing.
Sendy - Sendy is a self hosted newsletter app that sends emails 100x cheaper via Amazon SES
ListShine - Easy to use and affordable email marketing service with focus on simplicity and core features.