Thing is I find it very handy when logging in, but in this case it seems to be behaving oddly.
Even after I have saved the correct SMTP username , the field just shows the autofill username. Hence it appears that it isn't saving the correct one.
Also if you adjust some parameter on that page (e.g. minimum number of bounces) and then hit save, it changes the previously saved username to the default login autofill.
I did this yesterday, and for several hours email notifications stopped working. It was only by chance that I noticed the username was the wrong one.
I have not found a way for safari to disable this on specific pages, that would be good but ideally I don't think it should autofill if there is already another name saved in that filed.