Unity uses C#, so we were pretty confident it wouldn’t be an issue since there are so many .NET libraries around there. But none of them worked fine with Unity: some of them worked in Unity Editor, some were also able to work on Android, but none could pass Unity’s IL2CPP compilation for Xcode & iOS. We searched for solution on Unity’s forums but we find nothing, there was no other choice than testing all the libraries around the web!
During that time, we needed to be sure the project would be feasible. So we looked for alternative: a PHP solution for IMAP called Fetch combined with an AMF library for Unity. Not really a good replacement, but it worked.