Solved.
This is what I learned so far: once you've purchased a domain from Squarespace and an email workspace from Google, you have to add some new DNS (SPF) records manually. Without these valid SPF records, some of the sent emails were marked as spam by recipient email servers with strict policies.
However, I can only add one SPF record to a Squarespace domain so I had to merge the two SPF records into one as follows:
Create a new DNS record in Squarespace DNS zone:
type: TXT
host: @
value: v=spf1 include:_spf.google.com include:squarespace-mail.com ~all
This has seemed to have solved the issue.
