The IMAP certificate looks correct:
`openssl s_client -showcerts -connect mail.hispeed.ch:993 -servername mail.hispeed.ch
CONNECTED(00000003)
depth=2 OU = GlobalSign Root CA - R3, O = GlobalSign, CN = GlobalSign
verify return:1
depth=1 C = BE, O = GlobalSign nv-sa, CN = GlobalSign GCC R3 DV TLS CA 2020
verify return:1
depth=0 CN = mail.hispeed.ch
verify return:1
Certificate chain
0 s:CN = mail.hispeed.ch
i:C = BE, O = GlobalSign nv-sa, CN = GlobalSign GCC R3 DV TLS CA 2020
—–BEGIN CERTIFICATE—–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—–END CERTIFICATE—–
1 s:C = BE, O = GlobalSign nv-sa, CN = GlobalSign GCC R3 DV TLS CA 2020
i😮U = GlobalSign Root CA - R3, O = GlobalSign, CN = GlobalSign
—–BEGIN CERTIFICATE—–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—–END CERTIFICATE—–
Server certificate
subject=CN = mail.hispeed.ch
issuer=C = BE, O = GlobalSign nv-sa, CN = GlobalSign GCC R3 DV TLS CA 2020
—
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: RSA
Server Temp Key: ECDH, P-256, 256 bits
SSL handshake has read 3413 bytes and written 433 bytes
Verification: OK
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1.2
Cipher : ECDHE-RSA-AES128-GCM-SHA256
Session-ID: 94E3E60C09C67A91BDEBCC60B14F6259ED4C47A31BFBA41C9768F187005F32E9
Session-ID-ctx:
Master-Key: 6FECF4B3DBE7BAEEF033A08AF31A7D75B60913CCF0D026859FBAC523D49613AF9F44DA2E33D2DAB801E240426B31EF3F
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1659864459
Timeout : 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: yes
- OK [CAPABILITY IMAP4rev1 LOGIN-REFERRALS ID ENABLE AUTH=PLAIN] Dovecot ready.`
But I think it might be an Office 2007 issue, which doesn’t (anymore) support the current certificate standards. I suggest upgrading to at least Office 2019.