Sending email via Verizon static IP

Hi Experts,
I've got a situation that seems to have stumped Verizon 2nd level techs.

I have a client who regularly sends email from his on-site server through a failover device (a CradlePoint switches back and forth between two ISPs in the event of one failing). One side sends email fine, the other side is a Verizon cellular modem. During the time when failover is switched to Verizon and only during this time, sent emails are bounced back as spam (550 NDR) from 99.9% of all recipients.

I believe the PTR record needs to be changed at Verizon to associate outgoing email with client's email domain. Verizon is having trouble making this change.

I have already fixed up DNS on the client side to have dual static IPs (see my last posted question).

Does anyone know how to reliably send email via Verizon Wireless?
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Check the reverse resolution of the cellular IP.
What does the NDR say? Through and to which server are the emails being directed?
The mail server if this is a smarthost configuration on the internal mailserver, might not have the cellular IP authorized to relay.

The NDR should clearly state what the issue is.
Is it saying relaying denied?
Is it saying the source is being rejected because of spam type blacklist?
Is it saying that the queue lifetime has been exceeded and your own mail server generates the NDR to the server? This will point to the cellular IP not being allowed external connections destined to port 25. Cellular outgoing firewall rules preventing port 25 to anywhere other to ISP provider if at all...

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Elixir2Author Commented:
The NDR says the source is blacklisted - due to the fact that the IP address is correctly listed as a CELLULAR IP address. We have requested and received removal from that blacklist, but the fact remains that this process would have to be repeated for each recipient - on their end - their ISP would block it automatically due to the fact it is a cellular IP.

The suggested resolution by Verizon was to use a third party service to relay email. Verizon confirmed they DO NOT alter PTR records or any other aspect of their DNS for end users, just FYI.
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