How does delegate proxy server work with smtp?

Question

Email

We have a mailer located in one city, client applications (we collect and send mail through 1C) in another city. When sending letters from the 1C mail client, there is a delay, the connection to the mail server is slow. And the 1C server starts to freeze, not having time to send all the letters.
The task was to make an smtp proxy so that letters are sent through it, it is important that letters quickly leave 1C for a proxy in the same locale, and how they arrive to recipients is not important.
As such proxy it is possible to use delegate? Will this proxy pick up mail from the client and then forward it through the server? Or will he just replace his address with the server address, like stunel?
Or can you recommend something else to use?