Opera/Chrome客户端对目标端口有限制，参看

How to fix ERR_UNSAFE_PORT error on Chrome when browsing to unsafe ports - [2010-09-13]
https://superuser.com/questions/188006/how-to-fix-err-unsafe-port-error-on-chrome-when-browsing-to-unsafe-ports

Which ports are considered unsafe by Chrome - [2010-09-13]
https://superuser.com/questions/188058/which-ports-are-considered-unsafe-by-chrome


https://chromium.googlesource.com/chromium/src.git/+/refs/heads/master/net/base/port_util.cc

简单点说，在Opera GUI中输入"https://<FQDN>:<port>/"时，这个<port>有限制。port_util.cc中有个数组，kRestrictedPorts[]，定义了一堆端口号，当<port>位于其中时，会得到这样的提示

This site can't be reached

The webpage at https://<FQDN>:<port>/ might be temporarily down or it may have moved permanently to a new web address.

ERR_UNSAFE_PORT

ERR_UNSAFE_PORT提示是Opera客户端直接产生的，在本地产生的，连SYN包都不会发出去。加命令行参数可以绕过这个客户端限制

--explicitly-allowed-ports=<port,port,…>

比如

launcher.exe --explicitly-allowed-ports=25,465,587,993,2525

在其中访问

https://imap.gmail.com:993/

同时用Wireshark抓包，过滤imap.gmail.com解析出来的IP，可以看到发往993/TCP的SYN包。