Re: 553 sorry, that domain isn't in my list of allowed rcpthosts; no valid cert for gatewaying (#5.7
In addition to the things I mentioned above, if you are mail server administrator, make sure the person connecting has their IP address listed in the smtp allow rules.
If you are a user of the mail service, it sounds like you're hitting port 25 when you should be using another port such as port 587 with smtp authentication enabled.
If you did not make any changes to your settings it's possible you're using a mail service that turned off IP based smtp relay and now requires smtp authentication.
Another problem could be that you're trying to use a server that does allow IP based relay, but you're off network and they don't have alternate sending options.
What mail server name are you using in your outgoing smtp server settings?
What ISP are you connected to when you are getting this error message?