OK, just making sure your wallet is OK as well.
When I run:
[oracle@lin122 ~]$ openssl s_client -connect smtp.office365.com:587 -starttls smtp
CONNECTED(00000003)
depth=2 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
verify return:1
depth=1 C = US, O = DigiCert Inc, CN = DigiCert Cloud Services CA-1
verify return:1
depth=0 C = US, ST = Washington, L = Redmond, O = Microsoft Corporation, CN = outlook.com
verify return:1
---
Certificate chain
0 s:/C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=outlook.com
i:/C=US/O=DigiCert Inc/CN=DigiCert Cloud Services CA-1
1 s:/C=US/O=DigiCert Inc/CN=DigiCert Cloud Services CA-1
i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert Global Root CA
...
----BEGIN CERTIFICATE-----
MIIG/jCCBeagAwIBAgIQDs2Q7J6KkeHe1d6ecU8P9DANBgkqhkiG9w0BAQsFADBL
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMSUwIwYDVQQDExxE
aWdpQ2VydCBDbG91ZCBTZXJ2aWNlcyBDQS0xMB4XDTE3MDkxMzAwMDAwMFoXDTE4
...
lRDx4vTHGQiUzHY6EwgJ5iGRqgxUX/NmO4dJnC7IpKbdY/v1b9KKGzFpw27OkXqk
nGhseM2tJfwa2HMwUpuuo5029u4Dd40qvD0cMz33cOvBLRGkTPbXCFw24ZBdQrkt
SC5TAWzHFyT2tLC17LeSb7d0g+fuj41L6y4a9och8cPiv9IAP4sftzYupO99h4qg
7UXP7o3AOOGqrPS3INhO4068Z63indstanIHYM0IUHa3A2xrcz7ZbEuw1HiGH/Ba
HMz/gTSd2c0BXNiPeM7gdOK3
-----END CERTIFICATE-----
So you need to make sure that you
a) get/import the 2 certificates in the certificate chain into your wallet
b) import the base64 certificate as well.
If you're already done that, and you're still having dramas, it might be time to log a call with Support.