You can manually search for a record where you want to attach an email to.
👀 Watch this Gif or follow the steps below:

Step 1: Click Search for records.
Step 2: Choose from the dropdown list one of the supported record types you want to search for and enter the search criteria (company, name, invoice number etc.), and hit enter.
Choose the record you want to search for (1)
Type in the search criteria and hit enter (2)
Decide if you want to include attachments (3)
Check off the record(s) to attach it to (4)
Hit Attach (5)