I have been granted root access to one of the linode servers by my system administrator. I have a rails app running on apache server. Everything is fine. I also have a mail server configured through postfix. I have no problems sending e-mails from the server, except it goes to the spam box of the receiver. Googling this said I need to add SPF record like the following:
v=spf1 a mx ~all
But since I don't have access to the wen administrator interface provided by linode, I can't use linode's DNS Manager to add this record.
Is it possible to add this as a TXT record via command-line?
No, Linode's DNS hosting service is a completely separate service from the server to which you have access. Without access to the Linode Manager (https://manager.linode.com) or the Linode API (https://www.linode.com/api/dns) you can't update the DNS records.