The reason I ask is due to Barbara’s question about changing email provider as she has her own business domain, a web site and emails ending (say @xxx.com). If she changed email provider then using your analogy, then the hard-working mail team might be a team of contractors handling all her mail and then if she changes to a different provider then that would be equivalent to the old team being fired and new team of contractors put in their place. So same address, same building, same mail boy delivering it to your desk but a new mail room team. Would that be correct?
Your next major concern will be compatibility. It's not a shock that most businesses run on Microsoft Windows and use some form of Microsoft Office. Being able to use common third-party clients such as Microsoft Outlook can often be a concern, and even today, compatibility with Microsoft Outlook isn't necessarily guaranteed. This is especially true when sending and receiving meeting invites. It only takes one garbled meeting invite to realize how frustrating this can be in the real world. Even if using Microsoft Outlook isn't a concern, portability is. If the service is entirely web-based, then is there a means for me to take my email offline and send email when I connect?