One downside with ISP outbound relaying is SPF; if you want to use it you need to figure out how mail exits the ISP and keep the SPF records up to date; and eve

One downside with ISP outbound relaying is SPF; if you want to use it you need to figure out how mail exits the ISP and keep the SPF records up to date; and even if you do, suddenly all the other customers of that ISP can fake mail from you while passing SPF. Another downside is you lose logs of and insight in the mailq and the recipient smtp server responses.