You can do two things:

1. Configure Xeams not to raise these alerts
2. Fix the problem

Obviously, option 1 won't fix the problem but will simply hide it and therefore, is not recommended.

How to fix the problem

Before fixing the problem, it is important you understand why is this happening. Since your IP address is not allowed to send emails for @vendor-domain-name, emails going out from your IP address will be considered spam by the receiving end. There are two ways to fix this:

1. Ask the vendor to put your public IP in their SPF record. Once done, put the vendor's domain name as a local domain in Xeams. Note the change is needed in vendor's SPF, not yours.
2. Configure your in-house equipments to add a "Sender" header in the emails with an email address in your domain. Xeams will use the value in this header in SMTP communication and therefore, the receiving end will not treat it as forgery