I recently had to travel and use the internet in random locations without the availability of my carrier’s mobile signal. On some public Wi-Fi hotspots, I was unable to send an email.
Now of course I thought the problem lies with my server. Who in their right mind would block outgoing destination ports 465 and 587, right? Well, somebody did and, it’s a common malpractice. Even more surprising was when I learned that IMAP was blocked as well – only http/https and DNS traffic was allowed on the given network.
Why this is not a problem for most users
Most people use freemails like Gmail or Outlook, or their company uses Google Apps or Microsoft 365. So, they simply do not hit this problem as all the apps work over https.
When does this become a problem
When you (or more likely, your employer) has only IMAP allowed for emails. Meaning that you do not have an option to use a vendor-specific email app or EAS. IMAP is falling into obscurity these days.
What is the solution
While the obvious answer is using a service that has its own app (Outlook, Gmail, Zoho Mail, etc.), having a standards-compliant self-hosted email solution is still important. The only such service is Exchange Active Sync and is provided by multiple emailing/groupware solutions: Sogo, Horde, Z-Push (I do not know of any others, and they all have their own problems). For this reason, I use Sogo. Resorting to using EAS is the most viable option here – since there is no way to make sure all public Wi-Fi’s have IMAP and submission SMTP allowed. Or we could use a VPN but that seems like an overkill here if you don’t have any other use for it. I checked Cloudflare’s 184.108.40.206 app’s VPN, and it worked for the purpose of accessing IMAP and sending email on such network.