smtp.gmail.com から gmail-smtp-msa.l.google.com を調べる方法
host コマンド使えばいいらしい。
$ host smtp.gmail.com smtp.gmail.com is an alias for gmail-smtp-msa.l.google.com. gmail-smtp-msa.l.google.com has address 74.125.204.108 gmail-smtp-msa.l.google.com has address 74.125.204.109 gmail-smtp-msa.l.google.com has IPv6 address 2404:6800:4008:c00::6c
