ImprovMX doesn't host mail, it only forwards and sends it. Thus, there are no applicable incoming protocols (IMAP, POP, etc.) to be used with ImprovMX. Instead, the destination (forward-to of an alias) is responsible for hosting and receiving the incoming mail that ImprovMX forwards.
My mail client requires IMAP/POP authentication to send mail
Many mail clients will require incoming mail settings when attempting to add your custom domain alias as a sending address. If your mail client allows it, you can use fake or invalid incoming settings (mail won't be delivered on your alias anyway — it's forwarded to your destination) or you can utilize your destination address settings. I.E. if email@example.com forwards-to firstname.lastname@example.org, I would use email@example.com incoming settings, and firstname.lastname@example.org outbound/SMTP settings.
*In the scenario where the above is not valid, you'll want to seek alternative methods of creating a "send mail only" address instead of adding a full mail account. Here are a few common mail clients that allow for such usage: http://help.improvmx.com/en/?q=%22how+to+use%22
**Some mail clients (Apple's Mail for instance), do not easily allow the addition of these "send mail only" addresses. Consult your mail client documentation and/or support to test the tangibility of utilizing an ImprovMX "send mail only" address on your mail client. These types of aliases are often referred to as "external SMTP" aliases, "send mail only" addresses, "send mail as...", "SMTP accounts", and similar.
Still have questions?
We'll be happy to answer any questions you might have and help advise a solution for sending mail from your custom domain. Please let us know the following if you're requesting support:
What mail client are you attempting to send from?
Are you utilizing ImprovMX SMTP (Gmail SMTP will only work in Gmail)?
Are you receiving any errors, bounces, etc. when attempting to send or setup your alias?