A Subaccount can now send on an IP pool managed by the primary account

We have enhanced the header X-Mailgun-Sending-Ip-Pool to allow a subaccount to send via a primary account-level IP pool. This change was made to support a few different use cases but should remove the burden for primary account users to micro-manage IP allocation on the subaccount level.

We have enhanced the header X-Mailgun-Sending-Ip-Pool to allow a subaccount to send via a primary account-level IP pool. This change was made to support a few different use cases but should remove the burden for primary account users to micro-manage IP allocation on the subaccount level.