If you want to manage your mail.gr account through the Gmail interface to use advanced features:
and go to Settings (gear icon) > See all settings . Click the Accounts and Import tab.
On your computer, open Gmail. At the top left, click Compose. You can also open an existing draft. On the right of the "To:" line, Google Help Get Gmail features for your other email accounts - Computer
Enable this to group replies together, keeping conversations organized. 3. Mail Merge & Bulk Mailing (For Mass Communication)
Use labels in Gmail to categorize incoming mail. Create them on the left-hand menu, and use filters to apply labels automatically.
Go to Settings > Filters and Blocked Addresses to create rules that move, label, or delete emails automatically based on sender or keywords.
If you need to send personalized bulk emails from mail.gr (or a Gmail account): Send personalized emails with mail merge - Gmail Help
Under "Check mail from other accounts," click .