I don't know how to change ports, but will poke around.
Someone with the same program will be able to better advise you, but look for a checkbox etc. where you see the number 25 now for the port. Mine has a checkbox saying "override default SMTP port". Once it's checked, the number in the box (originally 25 as the default) can be changed to 587.
Say - I thoughts Macs were immune to all this stuff?
Ha. Ha. Ha. Alas, even the noble mac can't protect against the vagaries of the isp itself ....

We're not currently the same target for various attacks from undesirables as our windows brethren and sistren unless maybe we're accessing the net through a windows emulator (yes, my mac does Windows), but we're just as stuck with whatever the isp server decides to do to us.
I still can't send mail, so we are probably on different servers. Hope they get to mine by Monday, sending through a web interface is getting old fast. Also my computer is supposed to text me when client e-mail comes in, but can't do that now. So I have to run down the battery on my cell phone having it automatically check mail on my client-only account every 5 minutes.