Which SMTP port should I use, 587 or 465?

Either generally works, they just use encryption differently: 587 uses STARTTLS (starts unencrypted, then upgrades to TLS), while 465 is TLS from the very first byte of the connection. 587 is the more common modern default and what we'd recommend unless your provider specifically tells you to use 465. If 587 gets blocked on your network (some ISPs/networks block it to prevent spam relaying from compromised machines), 465 is a good fallback to try.