The subject line isn't probably that important - it's what is in the email that counts.
If you just say, 'I want to do a PhD in computational mathematics - do you have any opportunities', you're less likely to get a response that if you say 'I'm really interested in X topic, which your group works on'. Tailor each correspondence to the person you're writing to rather than using exactly the same text for each mail. Highlight your relevant qualifications and any useful research/work experience.
Good luck!