Delivery error 554 5.4.4 received by Tech's on cell phones

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We have 40 techs in the field. We email jobs from the AS400 to each tech's AT&T cell phone. Several times during the week, (no consistent) we will not receive the tech's response. Several hours after they tech replied to the email from us, they receive an undeliverable message "554 5.4.4 Internal Domain Look Up failed". This never happens with the tech's who reply with their gmail accounts. This is problem is becoming more frequent and causing frustration with the tech's. I have called AT&t Cell Department, Windstream - our internet provider...No one has a clue.
Hello, My guess would be the techs are temporarily out of internet service range, because the error 554 5.4.4 Internal Domain Look Up failed means that a domain name was unable to be resolved by the DNS system to an IP address that the message should be delivered to. Either that or there are problems with the mail server they are connecting to, and it's the one that is having difficulties looking up a domain name to send email to it. Unfortunately to help troubleshoot an issue like this typically mail logs would be used. Are the techs simply sending email directly out from their phones, or do they all have accounts setup on the same domain and are using a mail server to try to relay out their messages? - Jacob