Access to iCloud SMTP server via VPN service

I had been using a VPN service without problem for years. Recently, however, I found that I cannot connect to iCloud SMTP server ( smtp.mail.me.com ) thorough a VPN service any longer. I have contacted the VPN service provider and they confirmed that their configuration does not block access to SMTP service on smtp.mail.me.com. They suspect that the connection is blocked on iCloud SMTP server side.

Is there any way to re-enable access to iCloud SMTP server through a VPN service ?