This is not a very common occurrence, but it can be the case that an email thread that is associated with one customer contains emails from other customers that were included in the same email thread.
This issue may appear in the rare case that you are sending a single email to contacts across multiple customers (for example putting them on BCC).
Planhat will handle this case in the following ways:
the "Last Touch" will be updated for all of the companies that were included in the email
the email will be linked to all of the end users that were included in the email
but, the email conversation will only display in the activity log of the company that was first (an email conversation can only be related to/displayed on one company profile)
We recommend not sending emails as BCCs with multiple companies in general for a number of reasons. Either send the email from Planhat directly or if it's a huge list and this is more of a campaign email, then send via Mailchimp, Intercom or similar.