First: the isolating test

Log in to Webmail (cPanel → Email Accounts → Check Email) and send a message to an outside address, then reply to it. Webmail bypasses your phone and app settings entirely: if Webmail works both ways, your mailbox is healthy and the fault lives in an app's settings — compare them with the Mobile & Outlook guide.

Not receiving

• Mailbox or disk full: a full mailbox silently rejects new mail — and a full account disk stops everything. Check Email Accounts quotas and cPanel Disk Usage.
• DNS doesn't point here: if the nameservers or MX records live elsewhere, mail goes elsewhere. Changed hosts or DNS recently? That's the prime suspect (see the nameserver guide).
• It arrived — in spam/junk: check Webmail's spam folder before concluding anything is missing.
• A forwarder or filter is diverting it: review Forwarders and Email Filters for rules you — or a helpful colleague — set and forgot.

Not sending

• App settings: sending needs SMTP authentication ON, with port 465 (SSL) or 587 (STARTTLS) — the settings table in the Mobile & Outlook guide is the reference. "Receives fine, can't send" is this, nine times out of ten.
• Bounce messages are answers, not noise: the error text inside a bounce names the reason — a wrong address, a full recipient mailbox, a blocked message. Read it before retrying.
• Suspended account: an overdue invoice suspends email too. Billing → My Invoices tells the truth.

Sending, but landing in spam

That's a reputation and authentication problem — SPF, DKIM and DMARC first (the Deliverability guide), then sending habits (the Spam guide). Both live in the Email category.