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Hi All,

I have a problem that is getting quite frustrating. I recently moved from a Windows machine (using eMclient) to a Mac, using the default Mail client. All was going well up until 2 weeks ago, now every day I have an issue where I cannot send emails because Mail cannot establish an SMTP connection to the Mailcow server. IMAP and receiving emails is working fine. This is happening across all domains I have connected, 4 in total, but only to my machine, none of the other users on this Mailcow instance. Sending from SOGo always works fine.

Things I have tried:
-Check for an active ban on the Mailcow UI - always clear
-Reinstall the profile using the Apple config profile. I have tried both with a password included in the profile and without, same issue. However, removing the profile and installing it again allows me to send emails again for that day, then fails again the next day.
-Check with my ISP if for some reason they blocked a port.

So it has something to do with how my machine is connecting to Mailcow - but im not sure where to look or which logs to check for this. The Apple logs for Mail didn’t provide any clues that I could make out.

Any help appreciated.

Thanks

I think I have made some progress. The issue with the Mail client I think is the option in the screenshot. However I think that is a symptom of another issue. Now after some time it seems the Mailcow server is banning my home ip - but the odd thing is if I go to the UI via my phone, there is no active ban and my home IP is whitelisted. Apple Mail I think was rotating between ports to try and get around this “ghost” ban.

Looking at the net filter logs my ip never shows once - but I cannot access the server via the UI or in my mail client. I can SSH into the server no issue, so it must be at the Mailcow instance level. If I wait 30 min, I can access again for a bit, then appear to get banned.

I have no idea what is going on here.

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I think I made some more progress, doing a ping on required ports to the IP address worked no issue, however trying to ping via the hostname and ports did not work - my DNS is through Cloudflare. This would explain why it was acting like a ban, but I could not find anything on the Mailcow server and my IP was not in any log. Adding a firewall rule to Cloudflare seems to have resolved the issue, but only time will tell.

Another update, was not at the DNS level either. I’ve forced a new IP address at home - if something is blocking/banning an new IP should resolve for some time.

9 days later

A few days of error free running, so hopefully this answer helps someone with the same issue. The issue ended up being a DNS resolving issue and was occurring in 2 places.

1) Change your DNS settings on your home network. I have a Ubiquiti gateway and access point, it was using the system default, changing it to googles DNS cleared half the issue.

2) If you are on a Mac, it holds old DNS entries on the machine itself, so, combined with point 1, the machine was using old lookups to the wrong place. Run this to clear your Macs DNS cache.

sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder

Hope this helps.

2 years later

Thank you for your thorough analysis! I came here through google with exactly the same issue. For me, it seems like the culprit was the wrong SMTP port (Port 25 instead of 465) that MacOS Mail had configured automatically. After deactivating the “Automatically manage connection settings” and changig port to 465, it works flawlessly.

What I do not understand is why it worked for a few days.

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