Title says it all… Mailinabox has been running perfectly until just now and I cannot send mail. I don’t think I have made any changes other than updates. I can still receive perfectly fine and save drafts as just fine too. I have tried rerunning the setup script and nothing changed… 
Nevermind… it is just taking a very, very long time to connect to the server but it sends eventually. This isn’t great but the problem may disappear. 
EDIT: Nope it has gotten worse
It is at the point where it times out in all mail clients I have tried. It has gotten much worse. I am only capable of sending mail from round cube. Every mail client complains that it could not connect to the mail server’s SMTP outgoing server port.
I have checked fail2ban and there is no issue there. The only thing that has been banned in all of the jails is some unknown ips over ssh.
Here is the error that thunderbird in particular gives:
Sending of the message failed.
The message could not be sent because the connection to Outgoing server (SMTP) box.-------.rocks timed out. Try again.
Did you double checked your opened ports are the ones you need? At your server ufw and perhaps, the same ones opened at the custom firewall provided by your CLOUD server provider, too?
Updating mailinabox to the latest update that just came out today seems to have fixed the problem entirely.
Status check - not everything is green. Website domains are being resolved externally.
Domain’s email is directed to this domain. [xxxxx.com ↦ 10 box.xxxx.com]
✓
Postmaster contact address exists as a mail alias. [postmaster@xxxxx.com ↦ administrator@box.xxxxx.com]
✓
Domain is not blacklisted by dbl.spamhaus.org.
I am having exactly the same problem. Everything was working fanstastically.
On a mac using both airmail and spark. Everything was great, but now all of a sudden none of the clients can send mails. Test SMTP throws a similar error in each case:
‘Stable connection cannot be established’
Using the webmail client is fine, but no other client will work.
I’m on a Linode, don’t see any custom firewall.
I checked UFW.
To Action From
22 ALLOW Anywhere
53 ALLOW Anywhere
25/tcp ALLOW Anywhere
587 ALLOW Anywhere
993 ALLOW Anywhere
995 ALLOW Anywhere
4190/tcp ALLOW Anywhere
80 ALLOW Anywhere
443 ALLOW Anywhere
22 (v6) ALLOW Anywhere (v6)
53 (v6) ALLOW Anywhere (v6)
25/tcp (v6) ALLOW Anywhere (v6)
587 (v6) ALLOW Anywhere (v6)
993 (v6) ALLOW Anywhere (v6)
995 (v6) ALLOW Anywhere (v6)
4190/tcp (v6) ALLOW Anywhere (v6)
80 (v6) ALLOW Anywhere (v6)
443 (v6) ALLOW Anywhere (v6)`
Where do I check Fail2Ban?