SPF TXT records - do I need to include sub domains for an outsourced sender SPF record

Say that I want to send mail from my own servers at example.com.

As I understand it, I can specify the following SPF record and have any server addressed by an A record under example.com included by the SPF record. So evaluation of mail sent from mailserver1.example.com, mailserver2.example.com would result in a pass:

v=spf1 a ~all

If however I am using an outsourced mail sender say wesendmail.com, I would use an SPF record like this:

v=spf1 include:wesendmail.com ~all

My main question is - If wesendmail.com sends mail from servers at mailserver1.wesendmail.com, mailserver2.wesendmail.com, do I need to include additional SPF records for these servers, or are they captured by the above SPF record?

Also, if I do need to include additional records, how would I find out what their mail servers are? NSLOOKUP domain transfer attempts are blocked by most DNS servers.

Answer

The include clause in your SPF record fetches the corresponding SPF record from wesendmail.com and includes that in-line in your record. So assuming that wesendmail configures their record correctly, you don't need to do anything further.