How to solve the problem with the delivery of e-mail from Google to servers with verification through Realtime Blacklist (RBL)?

Question

Google

Good afternoon,
As a service for working with e-mail, our company uses google apps (an outdated free version), some of our partners use their own mail server with checking mail for spam through the Realtime Blacklist (RBL) service.
Our manager is constantly faced with the problem that letters do not reach the partner

Message will be retried for 2 more day(s)
Technical details of temporary failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain domen.com by mail.domen.com. [1.2.3.4].

The error that the other server returned was:
554 5.7.1 Service unavailable; Client host [209.85.161.194] blocked using bl.spamcop.net; Blocked - see http://www.spamcop.net/bl.shtml?209.85.161.194

Solutions attached by google https://support.google.com/a/answer/27642?hl=en
1. Just waiting until it disappears on its own is not suitable since the problem has been occurring for a long time and constantly (and only for this employee, what could be the reason for this behavior?)
2. Ask the recipient to add your domain name to the list of approved senders - The partner technical department replied that this is not possible
3. Ask him to add the Gmail IP addresses to the allowed list - We were told that they won’t do it, since this is a potential vulnerability for spam and that google can be hacked and they bombard them with spam (after that I gave up trying to convince their tech department of anything, because common sense is clearly not an argument)
Actually, the question is what method can be used to ensure that letters go through some specific (possibly third-party) SMTP only from specific mail accounts and only to the partner's mailing address (so that the entire mail service for the company is not changed)?
Is there any rational reason that letters from only one employee are checked and rebuffed (no one else in the company has encountered this problem)?

Answer 1

No.
Rather, you can ask Google to assign you to a dedicated IP or remove your IP from the spamcop database.
But with a very high probability you will not be heard.
> 1. and only for this employee, which may be related to such behavior
C by the fact that Google tied it to a specific server. Just wait until the IP disappears from the spamcop database.
If neither the mail service nor the technical services of the partner go to a meeting - either write from a personal box or use the send function on a different behalf - before large mailers had such a function - confirm the box, then you can go to Yandex and send a letter with [email protected] gmail.com.
Google's DMARK is not yet enabled - I don't know how it is on your domain. But you will have to send again through personal mail.
I would suggest such a solution - make your own mail server. Prescribe it in SPF. Send problem mail through it. (Set mail client to this address)