https://sendgrid.kke.co.jp/blog/?p=12756&utm_source=google&utm_medium=cpc&utm_content=text&utm_campaign=dsa&gad_source=1&gclid=Cj0KCQiA4NWrBhD-ARIsAFCKwWv9e5q63vQYH0xv-9W_IZAfBSPfPvkLbQiA8_ZCBflCsj0ZNwRBl64aArdfEALw_wcB
1. whois
$ telnet smtp.sendgrid.net 25