'Reply,' 'Reply All,' and 'Forward' buttons are not visible on emails in Lightning Experience

• The options are only available when Enhanced Email is enabled.
• Users can Reply, Reply All, and Forward if they are listed in the To or Cc fields in the email (for example, is a recipient) or are the sender.
• The Send Email global action is included in the global publisher layout assigned to the profile.
• The user isn't registered as a partner user in salesforce org.
• The options are available for Email Message records only. Users will not able to reply or forward emails logged as Tasks.
• When sending an email to a user record, the email is not logged to that user record. We need to send the email to a Salesforce record like lead or contact for the email to be displayed and logged in the Salesforce record.
• The email action must be mapped to the global publisher layout area 'Salesforce Mobile and Lightning Experience Actions' for Users to be able to view the Reply, Reply All, and Forward buttons in addition to the requirements mentioned in the article.

• You can't see the Reply, Reply All, and Forward standard quick actions on an Email.Message record if the sender's email address is an organization- wide address that isn't set for User Selection and Default No-Reply Address. Manage this setting in Organization-Wide Addresses in Setup.

Notes: 

• These options are available to the non-Case objects (for example, objects other than Case such as Accounts, Leads, Contacts, Opportunity, Custom objects, etc.).
• For Cases, Create a Send Email Quick Action for Cases, the custom email action in the Lightning Case Feed replies to the last email.
• These options are not available for emails that are sent through List or Mass Emails.

Here are a few things to know about how replying and forwarding works

• You can’t forward email attachments sent from Salesforce Classic in Lightning Experience.
• When forwarding emails, the most recent version of the file is attached rather than the version sent in the original email.
• Clicking 'Reply' or 'Reply All' opens the composer window and pre-populates the subject, previous message, and recipients of the previous message. Subject shows: 'Re: <old subject>' 
• Clicking 'Forward' opens the composer window and pre-populate subject, previous message, and attachment in previous message. Subject shows: 'Fw: <old subject>' 
• Crawled emails from Einstein Activity Capture don't have attachments, so attachments are only supported for emails sent from Salesforce.

Use the Workbench or Data Loader to understand how these options appear

• Write a SOQL query on the 'EmailMessage' object to identify if 'RelatedToId' shows non-Case object record ID (for example, Contact, Lead, etc.).

• Next, write another SOQL query on the 'EmailMessageRelation' object and verify if the Email contains a relation of any User record showing:

1. RelationAddress = User's email address
2. RelationId = User ID
3. RelationObjectType = User