Send email via secondary address

I created Excel VBA code to send emails on a loop through Outlook.

This works with my primary email address. When I change which email address it is sent from to my secondary email address, which is mapped over from Gmail, I receive an error message

This message could not be sent. You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x000004dc-0x00000524].

I cleared the cache, enabled third party apps in Gmail settings, and a few other things without success.

Application.ScreenUpdating = False

Dim i As Integer
Dim j As Integer

i = 3
Do While Cells(i, "A").Value <> ""
    emailaddress = Cells(i, 1)
    honorific = Cells(i, 2)
    lastname = Cells(i, 3)
    intro = xxx & xxx
    companytype = Cells(i, 4)
    firm = Cells(i, 5)
    customsentence = Cells(i, 6)
            
    'Replace placeholders within email body
    emailbody = Replace(emailbody, "Var1", xxx)
    emailbody = Replace(emailbody, "Var2", xxx)
    emailbody = Replace(emailbody, "Var3", xxx)
    emailbody = Replace(emailbody, "Var4", xxx)
    
    Set OutApp = CreateObject("Outlook.Application")
    Set OutMail = OutApp.CreateItem(0)
    With OutMail
        .sentonbehalfofname = SendAcct
        .to = emailaddress
        .cc = ""
        .Subject = emailsubjectline
        .Body = emailbody
        'Attachments.Add("") 'Placeholder for future attachments
        .send**