Webmail is the mail client that runs in your browser, served by the same server that holds the mailbox. Nothing to install and nothing to configure, which makes it useful for reading mail from a machine that is not yours, and considerably more useful as a diagnostic tool.
Getting in
Two routes:
https://yourdomain.com/webmail https://yourdomain.com:2096
Log in with the full email address and its password. The whole address, not just the part before the @: entering only the first part rejects a correct password, and it is the single most common webmail login failure.
You can also reach it from cPanel: open Email Accounts and use the Check Email link beside any mailbox, which signs you in without the password. That is the route to use when helping someone with their mail.
If the domain does not yet point at this server, the port 2096 form against the server hostname works regardless of DNS.
The diagnostic use, which matters more
Webmail talks to the mailbox directly, with no client configuration in between. That makes it the fastest way to split an email problem in half.
Someone reports mail not arriving. Open webmail. If the message is there, the mailbox is fine and the problem is the client's configuration or its filters. If it is not there, the message never reached the mailbox, and you look at MX records, quota and server-side filters instead.
That one check saves more time than anything else in email support, and it takes ten seconds.
The same applies to sending. If webmail sends and the client does not, the server is working and the client's outgoing settings are wrong, usually authentication switched off, or port 25 blocked by the network.
Roundcube
Roundcube is the interface cPanel provides. It handles the ordinary things well: reading, replying, attachments, folders, search, and an address book.
A few settings are worth changing once. Set your display name and a signature under Settings then Identities: without a display name your mail shows as a bare address, which looks like spam to recipients.
Under Settings then Preferences, the message list layout and preview pane arrangement are worth adjusting to how you actually read mail.
Folders are shared with your other devices
Webmail uses IMAP, so folders and read state are the same ones your phone and desktop client see. A message read here shows as read everywhere, and a folder created here appears on your other devices.
That is the point of IMAP, and it is also worth knowing before deleting things: a message deleted in webmail is deleted everywhere.
If a client is set to POP3 rather than IMAP, it may have already downloaded and removed mail from the server, in which case webmail shows an empty inbox while the mail exists on one laptop. That is a configuration problem worth fixing in place of a fault.
Searching for mail nobody can find
Webmail's search covers everything on the server, including folders a client has not synced.
When someone insists a message never arrived, search webmail for the sender's address before concluding anything. A surprising share of missing mail is sitting in a folder that a filter moved it to, or in spam, or in a subfolder the client never expanded.
Check the spam folder specifically. A server-side filter can move a message somewhere the mail client never looks.
Limitations
Webmail is not a replacement for a proper client if you handle a lot of mail. It has no offline access, and browser sessions time out.
Attachment size is limited by server settings, and a message that is fine from a desktop client can be refused here.
And it stores nothing locally, so everything you read stays on the server, which is correct, and which means the mailbox quota governs how much you can keep. Creating email accounts in cPanel explains setting that quota sensibly.
Security
Log out properly on a machine that is not yours rather than closing the tab, closing it leaves the session alive.
Use the https:// address. Over plain HTTP the mailbox password travels readable to anyone on the network path.
And treat the webmail login as a real credential. A compromised mailbox is not just read access: it becomes a spam relay, which gets the server's address blocklisted and affects every other site on it.
For reading the same mailbox on a phone rather than in a browser, How to Configure Email on Android deals with the settings.
Use it to prove where the fault is
Webmail runs on the server and talks to the mailbox directly, which makes it the fastest way to divide a mail problem in two.
If a message is visible in webmail and not in the client, the mail arrived and the fault is the client, its settings or its local copy. If it is not in webmail either, the message never reached the mailbox and the client is irrelevant.
That single check resolves most reports before any settings are examined, and it takes a minute. It also settles the more difficult conversation about whether mail was lost, because webmail shows what the server actually holds rather than what a client has synchronised.
Read the message source when delivery is in question
Webmail can show the full headers, and those answer questions nothing else can.
The received lines record each server the message passed through, in reverse order, with timestamps. Reading them from the bottom shows where the message originated and where any delay occurred.
The authentication results line says whether the sender's domain passed its checks. A message that failed those and arrived anyway explains why similar mail sometimes goes to spam, and a message from your own domain that fails them is a configuration problem on your side rather than the recipient's. Understanding email bounces and delivery codes deals with what the codes mean.
Check the filters before assuming mail was lost
Mail that arrives and then disappears is usually a rule doing exactly what it was told years ago.
ls -la ~/etc/example.com/user/ 2>/dev/null grep -riE 'deliver|discard|fileinto' ~/etc/*/ 2>/dev/null | head
Rules exist in two places and people only check one. There are account level rules and mailbox level rules, and a message can be moved or discarded by either.
A discard rule is the one that produces the report of mail vanishing without trace, since nothing is stored and no bounce is sent. Reading the rule list is the only way to find it, and on an inherited mailbox it is worth doing once as a matter of routine.