How to download the certificate of a smtp server

Run:

openssl s_client -starttls smtp -crlf -connect mail.example.com:25

Sample output:

openssl s_client -starttls smtp -crlf -connect  gmail-smtp-in.l.google.com:25
CONNECTED(00000003)
depth=2 C = US, O = Equifax, OU = Equifax Secure Certificate Authority
verify return:1
depth=1 C = US, O = Google Inc, CN = Google Internet Authority
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google Inc, CN = mx.google.com
verify return:1
---
Certificate chain
 0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=mx.google.com
   i:/C=US/O=Google Inc/CN=Google Internet Authority
 1 s:/C=US/O=Google Inc/CN=Google Internet Authority
   i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
---
Server certificate
-----BEGIN CERTIFICATE-----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==
-----END CERTIFICATE-----
subject=/C=US/ST=California/L=Mountain View/O=Google Inc/CN=mx.google.com
issuer=/C=US/O=Google Inc/CN=Google Internet Authority
---
No client certificate CA names sent
---