OK. I used several tests.
1) I sent them from my normal ISP (non gmail) account via SMTP to a sg alias. The alias is new and I have checked my sg account to make sure that is the case. The ISP is in Australia, TPG.COM.AU and I have verified the delivery speed from it to my sg forwarding address.
The new alias is jacksmith.d.<username>@neverbox.com
This is created and arrived.
2) I could send to existing aliases and arrived speedily.
3) From the same ISP using the same mailer, I tried to send
Jack.Smith.D.<username>@Neverbox.com
J.Smith.C.<UserName>@Neverbox.com
J.Smith.e.<UserName2>@neverbox.com
ForRegistration.D.<username>@neverbox.com
All are forwarded to the same gmail account that I have been using this service since 2005 and has not changed the forwarding address.
ALL ARE NEVER DELIVERED, ALL ABOVE NEW ALIASES are not created, except the jacksmith.d. I have never received any undelivered or bounce mail back from my ISP if this domain has been banned. I have recently changed to this ISP.
If neverbox.com has been banned, the mail I sent in item 1) would not have been delivered. Right? But it did.
4) I wish to disagree with your comment "It you attempt to send a test email from this
xyz@gmail.com account to one of your spamgourmet disposables, you will never see the test email."
I did send one from my xyz@gmail, my forwarding (protected) e-mail sg address, to a sg alias, adamsmith.c.<username>@neverbox.com, and it arrived. This is an already created alias.
5) I have just sent 2 e-mails to sg from hotmail they are both using the new aliases mentioned above. One to neverbox.com and one to spamgourmet.com
Neither of them arrived. I have checked the SPAM folder and there are not there.
The only conclusion I can come to is that there is some problem in creating new alias. I do not know why the jacksmith.d.<username>@neverbox.com is the only one that is created and delivered.
I will pm you my details to see if you can shed some lights on this issue. I have only discovered this when I tried to registered on a site and never receiving the activation mail.