If you don't have a mass emailing tool, you can still create a mail merge link using Outlook's mail merge feature - with the help of a few other Microsoft tools.
To start, first compose your email in Microsoft Word. Once done, open Excel to capture the fields that you'll need merged.
In your Excel file:
Start by adding three column names:
Email (cell A1)
DocSend_Link (cell B1)
Mail_Merge_Link (cell C1)
In column A, add the email addresses of your recipients (as many as you'd like). Then add your DocSend link in column B (ex. https://docsend.com/view/abc123).
Next, insert the following formula in column C, which is simply (1) and (2) combined together: =CONCATENATE(B:B, "?email=",A:A)
This will produce a custom link for each email address, such as https://firstname.lastname@example.org
Once this is done, highlight all the text and click "Sort & Filter" on the top-right. Click "Filter".
Now you're all set from Excel!
In your Word document:
Select the Mailings tab from the navigation bar at the top. From there go to the "Select Recipients" menu and choose "Use an Existing List".
From your documents, select the Excel file you just created.
You'll then want to click "Insert Merge Field" from the top ribbon and click "Mail_Merge_Link".
You can see from the screenshot below that the <<Mail_Merge_Link>> field has been inserted. Once you've previewed the merge and are happy with your email, select "Finish and Merge" and "Merge to E-Mail" to send your email!