If the Sending by Email Option is greyed out when setting up or managing a mail merge, follow these steps.
In the Admin Tool, click Mail Merges, then Edit the Selected Mail Merge.
To see what fields are available for the mail merge, in the Template and Merge Data section, click Change.
On the Query tab, ensure that an appropriate email field is selected.
Click OK to return to the Template and Merge Data section of the Mail Merge Properties window.
Click Fields.
Select the appropriate email field and click Set as E-mail Field.
Click OK.