leaky_bucket.py

from collections import defaultdict
from asyncio import Semaphore


MAX_CONNECTIONS_PER_MX = 1
leaky_bucket = defaultdict(lambda: Semaphore(MAX_CONNECTIONS_PER_MX))


async def ping_server_handler(request):
    ...
    ...
    
    async with leaky_bucket[mx_domain]:
        # only 1 await is allowed on smtp_handshake() at any time per every unique mx_domain
        await smtp_handshake(mx_domain, email)
    ...
    ...


async def smtp_handshake(mx_domain, email):
    ...
    await ..ehlo
    await ..rcpt
    ...