DataGridComboBoxColumn

 <DataGridComboBoxColumn Header="CustomersMail" SelectedValueBinding="{Binding oCustomersMail}" SelectedValuePath="Email" DisplayMemberPath="Email">
     <DataGridComboBoxColumn.ElementStyle>
         <Style TargetType="ComboBox">
             <Setter Property="ItemsSource" Value="{Binding ExistingCustomersEmails}" />
         </Style>
     </DataGridComboBoxColumn.ElementStyle>
     <DataGridComboBoxColumn.EditingElementStyle>
         <Style TargetType="ComboBox">
             <Setter Property="ItemsSource" Value="{Binding ExistingCustomersEmails}" />
         </Style>
     </DataGridComboBoxColumn.EditingElementStyle>
 </DataGridComboBoxColumn>

 <DataGridComboBoxColumn Header="CustomersMail" SelectedValueBinding="{Binding oCustomersMail}" SelectedValuePath="Email" DisplayMemberPath="Email">
     <DataGridComboBoxColumn.ElementStyle>
         <Style TargetType="ComboBox">
             <Setter Property="ItemsSource" Value="{Binding ExistingCustomersEmails}" />
         </Style>
     </DataGridComboBoxColumn.ElementStyle>
     <DataGridComboBoxColumn.EditingElementStyle>
         <Style TargetType="ComboBox">
             <Setter Property="ItemsSource" Value="{Binding ExistingCustomersEmails}" />
         </Style>
     </DataGridComboBoxColumn.EditingElementStyle>
 </DataGridComboBoxColumn>

public List<string> ExistingCustomersEmails
{
    get
    {
        // Example of a list of existing email addresses
        return new List<string> { "[email protected]", "[email protected]", "[email protected]" };
    }
}

public List<string> ExistingCustomersEmails
{
    get
    {
        // Example of a list of existing email addresses
        return new List<string> { "[email protected]", "[email protected]", "[email protected]" };
    }
}

internal class OrdView
{
    public string? oCustomersMail { get; set; }
}

internal class OrdView
{
    public string? oCustomersMail { get; set; }
}