Delay Receiving Emails from Outlook Clients

[wellington]

Hello,

I’m experiencing an issue with receiving emails from Outlook (Microsoft) clients.

The emails from Microsoft clients are sometimes delayed by 10 to 18 minutes, and in some cases, the delay can extend to several hours.

To mitigate this, we’ve added firewall rules to allow Microsoft IP ranges to connect for email purposes, as follows:

# Emails Microsoft
tcp|in|d=25|s=40.92.0.0/15 # Microsoft Exchange Online
tcp|in|d=587|s=40.92.0.0/15 # Microsoft Exchange Online
tcp|in|d=465|s=40.92.0.0/15 # Microsoft Exchange Online
tcp|in|d=25|s=40.107.0.0/16 # Microsoft Exchange Online
tcp|in|d=587|s=40.107.0.0/16 # Microsoft Exchange Online
tcp|in|d=465|s=40.107.0.0/16 # Microsoft Exchange Online
tcp|in|d=25|s=52.100.0.0/14 # Microsoft Exchange Online
tcp|in|d=587|s=52.100.0.0/14 # Microsoft Exchange Online
tcp|in|d=465|s=52.100.0.0/14 # Microsoft Exchange Online
tcp|in|d=25|s=104.47.0.0/17 # Microsoft Exchange Online
tcp|in|d=587|s=104.47.0.0/17 # Microsoft Exchange Online
tcp|in|d=465|s=104.47.0.0/17 # Microsoft Exchange Online
# Emails Microsoft END

However, the problem still persists. It’s also worth mentioning:

This issue is periodic — some days it occurs, other days everything works fine.

When testing, emails from Microsoft domains consistently show delays, but emails from Gmail clients arrive instantly without any issues.

If we restart the firewall, the problem is temporarily resolved, but the delays return after some time.

Does anyone have any suggestions on what else I should check? Could this be related to connection tracking, rate limits, or specific DirectAdmin settings?

Thanks in advance for any help.