Salesforce API calls

Key question… Is the bot:

• Creating records in the EmailMessage object only?

• Sending actual emails through Salesforce?

• Working against Cases (Email-to-Case) or against some custom object?

That distinction determines whether Lightning Threading, Email-to-Case, or email-header threading is the correct solution.

If the REST API call is successfully creating EmailMessage records, but each message appears as a new email record/conversation instead of being threaded into an existing chain, the issue is usually not the API itself, but rather the email threading metadata being sent to Salesforce. [help.salesforce.com], [simplysfdc.com]

What's likely happening

When you create an EmailMessage through the API, Salesforce treats it as a new email unless it can correlate it to an existing thread using:

• Message-ID

• In-Reply-To

• References

• Lightning Threading Tokens (for Email-to-Case and modern threading) [simplysfdc.com], [salesforcetime.com]

Simply inserting a new EmailMessage record with the same Case, Contact, or Account does not automatically make it part of the same email conversation. [simplysfdc.com], [developer....sforce.com]

Recommended approaches

Option 1 (Best for Case-related email conversations)

Use Email-to-Case with Lightning Threading.

Salesforce's current recommendation is to use the newer Lightning Threading model, where outbound emails include a threading token and inbound replies are matched back to the original Case. [help.salesforce.com], [salesforcetime.com]

This is generally the most reliable solution if the emails belong to a Case lifecycle.

Option 2

Populate the email header fields correctly when creating the email:

{
"MessageIdentifier": "<new-message-id@domain.com>",
"ReplyToEmailMessageId": "02sXXXXXXXXXXXX"
}

• In-Reply-To

• References

headers from the previous email chain. Salesforce uses these values for header-based threading. [simplysfdc.com]

Option 3

If the bot is sending outbound emails and later appending updates, consider using:

• EmailMessageRelation

• ParentId (Case)

• Existing EmailMessage references

rather than creating standalone EmailMessage records each time. However, this associates records—it does not by itself create true email threading. [developer....sforce.com]

APIs I'd recommend

1. Messaging / Email-to-Case APIs with Lightning Threading (preferred) [help.salesforce.com], [salesforcetime.com]

2. EmailMessage REST/SOAP API with proper header values (In-Reply-To, References, Message-ID) [simplysfdc.com]

3. If Service Cloud is involved, use the Case + EmailMessage model rather than directly creating unrelated email records. [developer....sforce.com], [help.salesforce.com]