Mapping custom fields in CRM email parser

Mapping custom fields in CRM email parser

Hi there, 

I've got a form on my site that currently integrates with Zoho via API, but I'm hoping to replace that integration with an email parser and push leads to CRM that way.

Sadly it seems that Zoho's email parser cannot map custom fields.  In the email parser interface all of the default fields show up fine, but my custom fields do not show up at all.  This is pretty disappointing.  

Is there any way to map custom fields in the email parser?  

Thanks