我尝试从gopkg.in安装不同的软件包，但它们都不起作用。
示例：
$ go get -v gopkg.in/gomail.v2
Fetching https://gopkg.in/gomail.v2?go-get=1
Parsing meta tags from https://gopkg.in/gomail.v2?go-get=1 (status code 200)
get "gopkg.in/gomail.v2": found meta tag get.metaImport{Prefix:"gopkg.in/gomail.v2", VCS:"git", RepoRoot:"https://gopkg.in/gomail.v2"} at https://gopkg.in/gomail.v2?go-get=1s
gopkg.in/gomail.v2 (download)
# cd .; git clone https://gopkg.in/gomail.v2 /data/gowork/src/gopkg.in/gomail.v2
Cloning into '/data/gowork/src/gopkg.in/gomail.v2'...
fatal: unable to connect to gopkg.in:
gopkg.in[0: 45.33.37.13]: errno=Connection refused
package gopkg.in/gomail.v2: exit status 128
我的git版本是2.13.0，我用
启用了git sslCertify$ git config --global http.sslVerify true
没有防火墙或代理阻止与gopkg.in上的SSL端口的连接：
$ nmap gopkg.in -p 443
Starting Nmap 7.12 ( https://nmap.org ) at 2017-10-13 12:54 CEST
Nmap scan report for gopkg.in (45.33.37.13)
Host is up (0.25s latency).
rDNS record for 45.33.37.13: 13.37.33.45.in-addr.arpa
PORT STATE SERVICE
443/tcp open https
Nmap done: 1 IP address (1 host up) scanned in 0.53 seconds
有任何线索吗？