The Communication tab collects everything that has been exchanged with a customer in one place. It is split into two sub-tabs: Internal, a chronological feed of emails, notes, and tickets, and Messengers, the list of messenger chats such as WhatsApp.
Conversations are private: an email thread between an administrator and a customer is visible only to the sender and the recipient, so other administrators cannot read it in Splynx.
The tab works the same way for customers and leads.
Customers: Customers → List → [customer] → Communication:

Leads: CRM → Leads → [lead] → Communication:

To send emails from the tab, the customer needs a valid email address and the SMTP server has to be configured. Set up email communication in the admin's Profile section on the Email tab:

For more information, read Email (Profile), Email (Config), and Messages.
The Messengers sub-tab requires a messenger integration to be configured first — see WhatsApp.
The feature is available for both customer and lead profiles.
The Internal sub-tab lists all interactions with the customer, newest first. Emails, notes, and tickets appear in the same feed, so the whole history is visible without switching pages.
Each entry in the feed shows:

Narrow the feed down by date, by interaction type, or by text.
Filter by period — pick a date range in the calendar to show only interactions from that period:

Type — use the dropdown next to the period filter to show a single interaction type instead of All:

Search interactions — type into the search field above the feed to find interactions by subject or message text.
To write to the customer, click Send email in the top right corner of the sub-tab:

Specify the email addresses and the subject, attach files, and insert a canned response if needed:

The 'Send email' button is inactive when the customer has no valid email address or when the admin's email settings are not configured.
The Messengers sub-tab lists the messenger chats held with this customer — for example WhatsApp conversations — so the chat history stays on the customer profile alongside emails and tickets.
The table contains one row per chat:

Use the refresh button above the table to reload the list without reloading the page.
To contact the customer in a messenger, click Create chat in the top right corner of the sub-tab. In the Create new chat window, fill in:
Click Create to open the chat, or Close to discard it:

Messenger chats are only available for customers whose phone number is reachable in the configured messenger. Configure the integration first — see WhatsApp.