I am an experienced Linux administrator.
I can authenticate myself so you could be sure that I will do no harm to your server / services nor abuse your data.
Also, I am ready to sign NDA if needed. I can be verified for example by VAT EU number.
Please provide more details so then I could estimate time and effort needed for this task and adjust my bid accordingly.
I'm not sure how serious you are with your project, but I don't think that your budget is even close to what is required for only a small part of this task.
I could help you as an system architect and system administrator - maintaining server infrastructure and back-end services (smtp server, imap server, database server, etc.)
Roundcube is small but lightweight and robust, most probably preferred for your case, but this is only a front-end. Most probably you would need to hire programmer (I could also help you with that but I'm not that effective as full-time programmer could be) to make some more advanced admin panels, additional plugins with required features / add-ons, etc.
Please consider re-posting this project on a hourly rate basis or even full-time if you are seriously thinking about this kind of service. Running such as 'fire and forget' would only make a trouble (quickly it became a spam source and would be placed on blacklists)
cheers,
Szymon Łapiński