From: Tom Hendrikx on
Bob Eastbrook wrote:

> NOQUEUE: reject: RCPT from
> web81307.mail.mud.yahoo.com[68.142.199.123]: 554 5.7.1
> <bob(a)myapp.appspot.com>: Relay access denied;
> from=<a-yahoo-user(a)yahoo.com> to=<bob(a)myapp.appspot.com> proto=SMTP
> helo=<web81307.mail.mud.yahoo.com>
>

This says that the yahoo user tries to send mail addressed to
bob(a)myapp.appspot.com, not to bob(a)example.org.

Your mail server is not configured to accept mail for that domain (but
DNS records point to it), so either:
- do not send mail to *@myapp.appspot.com
- add myapp.appspot.com to $mydestination

Regards,
Tom