Let’s consider a use-case of this problem:

The customer is using email2case. When customers send an email to the support email ( like product@domain.com) then a case is successfully created. Now, from Console when an Agent replies back to the customer from Feed->Send mail the email is NOT reaching the customer.

Solution:

1. Create DKIM key in Salesforce

https://help.salesforce.com/s/articleView?id=emailadmin_create_secure_dkim.htm&type=5&language=en_US

2. publish the CNAME and alternate CNAME to the DNS server. You may need to involve your infrastructure IS and email domain admin

3. Once done, wait for few hours and activate the DKIM key in Salesforce. 

4. Next is to have Salesforce SPF added in your DNS’s SPF records. Below is the article for reference:
Include Salesforce in Your SPF Record
https://help.salesforce.com/articleView?id=sf.emailadmin_spf_include_salesforce.htm&type=5

5. Once you have activated DKIM and SPF, we recommend you to disable the below deliverability settings:
– ‘Activate bounce management’
– ‘Enable compliance with standard email security mechanisms’