Which host will be the one actually "sending" mail on behalf of the domain? That's what you need to add to the SPF record.
In the case of "mxpool.example.com" sending mail on behalf of [email protected], this will work:
example.com. IN TXT "v=spf1 mx a ptr a:mxpool.example.com -all
'mx' allows the host listed as the mx
"a" allows the IP of the "A" record (1.2.3.4) to send mail on behalf of example.com
"ptr" allows all subdomains of example.com to send mail on behalf of example.com (it's fine as long as you control the entire domain)
"a:mxpool.example.com" explicitly authorizes that host to send mail on behalf of example.com (even though it falls under the two preceding rules).
and "-all" tells other mail servers that mail should be rejected (fail SPF) if it does not come from one of the authorized hosts.
EDIT:
If you want hosts belonging to OtherSendingDomain.com to send on behalf of yourdomain.com you can add an "include" to your spf record. Should work just fine.
yourdomain.com. IN TXT "v=spf1 mx a ptr a:mx.yourdomain.com include:OtherSendingDomain.com -all"
The way SPF works, you need to know and allow every host which will be sending mail on your domain's behalf. They can be explicitly stated in the spf record in some fashion, or inherited/included from another domain.