A shared inbox is an excellent first system. It costs nothing, everyone already knows how to use it, and for a while it genuinely works.
Then one day a request gets missed because two people each assumed the other had it, and the conversation about a portal starts.
The signals that you have outgrown it
People ask “did anyone answer this?” more than once a week. That is a status problem, and email has no status.
You cannot answer how many open requests exist right now without reading the inbox. If the count requires scrolling, you cannot manage the workload or staff against it.
The same attachment gets requested twice because nobody can find the first copy. Email is a terrible filing system and an even worse search index.
Clients ask for updates by email, which generates more email, which is the thing you were already behind on.
What a portal actually needs to do
Far less than most first drafts assume. Submit a request with the right fields attached. See the status of everything open. Find past documents without asking. That is most of the value.
What it does not need in version one: messaging, notifications for every event, dashboards, a mobile app, or a permissions matrix with nine roles. Each of those is defensible on its own and collectively they are why portal projects stall.
The part teams get wrong
A portal only works if it is the path of least resistance. If a client can still email you and get the same result, they will, and you now run two systems instead of one.
That is an operational decision, not a technical one, and it has to be made before the build rather than discovered after launch.
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