Automapper, mapping single destination property as a concatenation of multiple source property

One possible way to achieve this is to create a map that uses a specific method for this conversion. The map creation would be:

Mapper.CreateMap<EmailViewModel, Email>()
    .ForMember(e => e.EmailRecipient, opt => opt.MapFrom(v => JoinRecipients(v)));

Where the JoinRecipients method would perform the conversion itself. A simple implementation could be something like:

private ICollection<EmailRecipient> JoinRecipients(EmailViewModel viewModel) {
    List<EmailRecipient> result = new List<EmailRecipient>();
    foreach (var toRecipient in viewModel.To) {
        result.Add(new EmailRecipient {
            RecipientEmailTypeId = 1, 
            RecipientEmailAddress = toRecipient.RecipientEmailAddress

    foreach (var ccRecipient in viewModel.Cc) {
        result.Add(new EmailRecipient {
            RecipientEmailTypeId = 2,
            RecipientEmailAddress = ccRecipient.RecipientEmailAddress

    return result;

I'm a huge opponent of converters, mostly because for other people in your project, things will just happen 'like magic' after the mapping call.

An easier way of handling this would be to implement the property as a method that converts other properties on the viewmodel to the required formatting. Example:

public class EmailViewModel
    public ICollection<EmailRecipient> EmailRecipient { 
        get {
             return To.Union(Cc);
    public List<EmailRecipientViewModel> To { get; set; }
    public List<EmailRecipientViewModel> Cc { get; set; }
    public string Subject { get; set; }
    public string Body { get; set; }

Now automapper automatically maps from EmailRecipient property to EmailRecipient property, and if someone is trying to figure out how it happens, they just need to look on the viewmodel.