$ openssl s_client -connect imap.ge.xfinity.com:993 -crlf
CONNECTED(00000003)
depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority
verify return:1
depth=1 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Organization Validation Secure Server CA
verify return:1
depth=0 C = US, postalCode = 19103, ST = PA, L = Philadelphia, street = 1 Comcast Center, O = Comcast Corporation, OU = NSO, OU = Hosted by Comcast Corporation, OU = Unified Communications, CN = imap.email.comcast.net
verify return:1
---
Certificate chain
0 s:/C=US/postalCode=19103/ST=PA/L=Philadelphia/street=1 Comcast Center/O=Comcast Corporation/OU=NSO/OU=Hosted by Comcast Corporation/OU=Unified Communications/CN=imap.email.comcast.net
i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Organization Validation Secure Server CA
1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Organization Validation Secure Server CA
i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
2 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
3 s:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=/C=US/postalCode=19103/ST=PA/L=Philadelphia/street=1 Comcast Center/O=Comcast Corporation/OU=NSO/OU=Hosted by Comcast Corporation/OU=Unified Communications/CN=imap.email.comcast.net
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Organization Validation Secure Server CA
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: ECDH, P-384, 384 bits
---
SSL handshake has read 6845 bytes and written 334 bytes
Verification: OK
---
New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384
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-AES256-GCM-SHA384
Session-ID: E7AF27584A61019C12D234904A4914971B67EDFE4A17D698B84D7C2411536600
Session-ID-ctx:
Master-Key: DF440ABCF15B1CE2466C3BF7BD2854C8E03CDACF7DA494FC0707B150D6B5002F752B681BC0732AF2AAF22D7E8DB7586F
PSK identity: None
PSK identity hint: None
SRP username: None
TLS session ticket lifetime hint: 300 (seconds)
TLS session ticket:
0000 - f8 c2 9d a0 58 3b 28 3a-97 3d 68 89 af 8c c2 cd ....X;(:.=h.....
0010 - dd ff bd 27 6d 91 92 cc-ca 6f d2 bc 57 5b 93 e0 ...'m....o..W[..
0020 - 26 3f aa e9 c3 ee e2 57-33 8e 71 a5 ba 5e 9a cf &?.....W3.q..^..
0030 - aa d5 18 f7 9b 44 21 e6-f1 8b 18 a1 f3 04 15 b6 .....D!.........
0040 - 14 eb 58 91 31 b5 8e 7c-e5 87 b5 c9 c3 87 72 5f ..X.1..|......r_
0050 - 6e 84 43 c6 78 98 b4 a3-11 89 3f 08 b4 2a d5 9a n.C.x.....?..*..
0060 - 8e c0 14 fa 00 af 41 e2-27 f4 f4 11 31 91 86 3d ......A.'...1..=
0070 - 70 3e 07 62 28 ab 88 29-64 69 e1 d7 ea 6c c2 a6 p>.b(..)di...l..
0080 - 5f 23 f7 58 ce ff 43 fc-75 0e ae 5d dd f1 10 70 _#.X..C.u..]...p
0090 - 5b 25 e8 0a eb 93 48 95-a5 69 74 a5 8f 3d 14 9f [%....H..it..=..
Start Time: 1545599590
Timeout : 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: no
---
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE AUTH=PLAIN AUTH=LOGIN AUTH=OAUTHBEARER AUTH=XOAUTH2] Dovecot ready.
Again, thank you all for your help. As you can see, all of these checks work properly, but still roundcube is not working.