Fastest Method:

Here's a method much faster and more compact than any of the other answers, and works with numeric or text values:

Function IsIn(valCheck, valList As String) As Boolean  
    IsIn = Not InStr("," & valList & ",", "," & valCheck & ",") = 0
End Function

Examples:

Use IsIn with a numeric value:

Sub demo_Number()
    Const x = 2
    If IsIn(x, "1,2,3") Then
        Debug.Print "Value " & x & " was Found!"
    Else
        Debug.Print "Value " & x & " was not Found."
    End If
End Sub

Use IsIn with a string value:

Sub demo_Text()
    Const x = "Dog"
    If IsIn(x, "Me,You,Dog,Boo") Then
        Debug.Print "Value " & x & " was Found!"
    Else
        Debug.Print "Value " & x & " was not Found."
    End If
End Sub

Speed Comparison:

To compare speed I ran the test from the accepted answer 100,000 times:

• 0.406 sec (FASTEST) This Function (using InStr):
• 1.828 sec (450% slower) Accepted Answer with the "ISIN" function
• 1.799 sec (440% slower) Answer with the "IsInArray" from freeVBcode
• 0.838 sec (206% slower) Answer with modified "IsInArray" function

I didn't include the much longer answer that uses SELECT..CASE since the OP's goal was presumably to simplify and shorten the task compared to "if x=1 or x=2 or x=3 then".

It doesn't work without writing your own function. Be aware that the accepted solution by @Kredns may not work as expected for all types of objects since they are coerced to strings (which also may raise Type Mismatch errors).

This solution should (hopefully) handle all types of data (at least in Excel 365, not sure about earlier versions):

Function IsIn(x As Variant, list As Variant) As Boolean
    ' Checks whether list (Array) contains the element x
    IsIn = False
    For Each element In list
        If x = element Then IsIn = True
    Next element
End Function

dim x, y
x = 2
y = Array(1, 2, 3)

For i = 0 To 2
If x = y(i) Then
'your code comes here
Exit For
End If
Next i

I wrote it now...

Public Function IsInArray(FindValue As Variant, ParamArray arrEmailAttachment()) As Boolean

Dim element As Variant

For Each element In arrEmailAttachment
    If element = FindValue Then
        IsInArray = True
        Exit Function
    End If
Next element

IsInArray = False

End Function