I have ads at Craigslist.org that use Spamgourmet aliases in the format:
<adname>.<myaccount>@xoxy.net
where,
<adname> is something to remind me for which ad a persons is sending an email inquiry. It only contains alphanumeric characters (always starting with alphabetic and may end with a numeric string, like today's date). No non-alphanumeric characters are used.
<myaccount> is my account (username) at Spamgourmet.
xoxy.net is one of the domains pointing at Spamgourmet (and a lot shorter to enter).
I have 5 ads at Craigslist and tested the Spamgourmet alias at each one. The first 4 worked without incident; that is, I got my test e-mail using the alias. The 5th alias doesn't work: the test e-mail was not received at the real e-mail account. I tried sending another test e-mail to the 5th alias and still never got the test e-mail at my real e-mail account. The usage counter for the alias decremented each time so Spamgourmet got the test e-mail and then tried to send it (forward). Yet my real e-mail account never gets the test e-mails for the 5th alias.
The test e-mail is not getting filtered by a server-side spam filter (i.e., it's not getting dumped into the Spam/Junk folder). I am using the webmail interface to my real e-mail account to avoid any client/proxy on my computer from filtering out the test e-mails. The test e-mails are not going into the Trash folder, either. They aren't showing up in any webmail folders. My server-side rules do not 'discard' any incoming emails but, at most, move them into the Trash folder.
I doubt my real e-mail provider's SMTP server rejected my test e-mails. They were successfully received through the first 4 aliases. My e-mail provider doesn't employ greylisting to force senders to resend their e-mails (valid SMTP servers will retry but spam-controlled SMTP servers will not). If Spamgourmet's SMTP server encounters an error, it should get logged and the server should retry later (but it isn't).
This isn't really a unique or rare event. Many times I've wondered why I didn't not received expected e-mails through an alias. I used to use Sneakemail but they yanked the free service tier and SpamMotel sucks so Spamgourmet is about the only one left for free service. Spamgourmet users cannot access a log showing what happened between Spamgourment and the SMTP server to which it tried to send the e-mails through an alias. I don't know if Spamgourmet itself keeps logs of its SMTP sessions (for sending to the real e-mail address, not for receiving to the alias at Spamgourmet).
I can try using a different alias at Spamgourmet to see if that works. Because Spamgourmet is less than 100% reliable for receiving e-mails sent to Spamgourmet aliases. I've gotten into the habit of sending a test e-mail to make sure it works (well, that it works at least one time). SMTP logging should be employed so Spamgourmet can determine at what point during an SMTP session there was problem. Operating blindly (no logs) means Spamgourmet hasn't a clue why an SMTP session failed and if it is a problem with the other end or on Spamgourmet's end.