imap tls problems

Can't get Imap working over tls

on https no problems:

s_client -connect mydomain.com:https

CONNECTED(00000005) depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3 verify return:1 depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3 verify return:1 depth=0 CN = mydomain.com

verify return:1

Certificate chain 0 s:CN = mydomain.com   i:C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3 1 s:C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3

  i:O = Digital Signature Trust Co., CN = DST Root CA X3

on imap: openssl s_client -connect mydomain.com:imaps

CONNECTED(00000005) depth=0 CN = mydomain.com verify error:num=20:unable to get local issuer certificate verify return:1 depth=0 CN = mydomain.com verify error:num=21:unable to verify the first certificate

verify return:1

Certificate chain 0 s:CN = mydomain.com   i:C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3

Derver debian 9 with 6 domains.