Add setting SMTP header by inbox#21

Cannot use multiple inboxes when sending emails with Sparkpost via SMTP. We could if we could set a header for each inbox.

For example, if using Sparkpost, we need the ability to set which IP pool to use per inbox. Like this:

X-MSYS-API: { “options”: { “ip_pool”: “my_new_ip_pool” } }

See docs here:

https://support.sparkpost.com/docs/deliverability/dedicated-ip-pools#using-your-dedicated-ips