Mentions (@)

Type @ and the first letters of someone's name to mention them, as in Outlook or Teams. A list of matching people appears; choose one with the mouse, or with the arrow keys and Enter (Escape closes it).

In mail

In webmail's composer, mention teammates (people on your team) and your contacts. The mention is written as a link reading @Name to their address, and they are added to To if they are not a recipient yet, so the message reaches them.

When a message mentions you, it is marked for you: webmail's Mentions view (under your folders) lists every message that mentions you, whatever folder it is in. Mentions from Outlook and other mail apps that write them the same way count too. Only your copy is marked; the other recipients' are not.

In the calendar

Mention someone in an event's notes and they are added to its Guests; when you save, they get an invitation (see Calendars).

In notes

Help desk (Zappocity panel): in an internal note on a ticket, mention a staff member; they get an email saying so, with the note. Mentions in replies to the customer notify nobody.
Notes on an account (the account's Notes, in either panel, or the domain admin panel): mention someone who can read the note: staff on a staff-only note, the team's admins on a team note. They get an email.

Only people allowed to read a note are told about it; mentioning anyone else changes nothing.

For developers

Delivery marks a message with the keyword $mentioned for a recipient when its HTML has a mailto: link to one of their addresses whose text starts with @; JMAP Email/query with {"hasKeyword": "$mentioned"} finds them, and IMAP shows the keyword as a flag.
Notes take mentions, a list of email addresses: POST /v1/platform/support/tickets/{ticketId}/messages (internal notes; staff only) and POST /v1/tenants/{tenantId}/accounts/{accountId}/notes.