SetFeild [yourfield, self & "Mother's Day Email]
If you are just trying to add that text to the field. It seems like there are better ways of doing that though. If the correspondence was in a separate table, you could just link each correspondence to which ever contacts you wanted through a join table.
Use the calculation option in Replace Field Contents with an expression such as:
Marketing & " Mothers Day Email"
There are a number of possible expressions that can be used to combine the new text with the old.
List ( Marketing ; "Mothers Day Email" )
Puts the new text on it's own line of text.
But appending multiple text entries into a list in the same field is often not the best design option for recording such data. Setting up a portal to a related table where adding more data is done by adding a new record in the related table is often a more flexible way of handling the same data.