I am using Zulip, a Slack-like messaging tool. It has the option of being able to respond to messages in the app via email. For that purpose it creates alias email adresses like this for each channel
Actually it does support the + alias mechanism. But perhaps postfix chokes on the “.” or the “-” in the email? Check /var/log/mail.log to see if there are more details on the error there.
Update: duckduckgoing this a bit, it might not have anything to do with the default mailinabox installation. Did you have to do any changes to the configuration to get Zulip to work?
I tried it on my box by sending mail to user+6feab70a1cc46b8586feff156570a20e.show-sender@mybox.com which works without a problem.
I wonder if the zulip address is a user/mailbox or an alias? Perhaps there are already aliases defined on your box that interfere? E.g. a domain alias or a aliases that refer to each other?
Indeed! zulip@mybox.com was an alias of noreply@mybox.com and vice-versa. I do not remember why that was there. It may have been necessary during the transition of one messaging app to another.