probably the wrong thread to ask but what should a 5-10 person company with 20-25 outside contractors and lots of costumers that may need to access the issue use? I'm currently consulting for a company who has no issue tracking system. they use an excel sheet which makes it impossible to have conversation about individual issue and nearly impossible to track updates. I can't imagine getting them to spend $300+ a mont…
There may be provider-specific productivity features, such as G Suite's Collaborative Inbox, or Exchange's Public Folders. The IMAP Protocol also supports shared mailboxes, so any IMAP server that supports this will allow you to collaborate on issues sorted into folders that multiple users can access. Some open source mail servers definitely support it. Once they pick up a real issue tracking system it will probably support issues created via e-mail, so it can be seamlessly integrated later.