Comcast SMTP server problem

try using either port 465 or 587 if you want to use STARTTLS or SSL/TLS

Yes, the correct port is used for each protocol (if you have not not use Thunderbird, the software is smart enough to set the default port when a certain protocol is selected).

I can telnet directly to the port numbers on the server.  So this could be a problem with Thunderbird.

My coworker ran into the exact same problem after updating Thunderbird.  So this seems to be a compatible issue with the latest version of Thunderbird.  Just wonder if anybody at comcast has time check it out?  Thunderbird is freely available and there should be quite a few of people using it (not really based on the responses here!).

I'm having the same issue, but I don't think this is a Thunderbird problem. I do not think that Comcast doesn't seem to be supporting TLS on thier business class SMTP outgoing email servers. The older client may have been falling back to un-encrypted mode. On a side note, the residential service (Xfinity) server does support tls. I looked for a way to submit and support ticket on this issue but only found this forum. I doubt I could get my point across over the phone of what the problem is. Here's how I tested on my Arch Linux box using openssl. 

[tsweets@keystone ~]$ openssl s_client -connect smtp.w14a.comcast.net:587 -starttls smtp
CONNECTED(00000003)
140715379508888:error:140790E5:SSL routines:ssl23_write:ssl handshake failure:s23_lib.c:177:
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 273 bytes and written 353 bytes
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated

HOWEVER

The residential service server (smtp.comcast.net) does

[tsweets@keystone ~]$ openssl s_client -connect smtp.comcast.net:587 -starttls smtp
CONNECTED(00000003)
depth=2 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify return:1
depth=1 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO High-Assurance Secure Server CA
verify return:1
depth=0 C = US, postalCode = 19103, ST = PA, L = Philadelphia, street = 1 Comcast Center, O = Comcast Corporation, OU = Business Center, OU = Hosted by Comcast Corporation, OU = EliteSSL, CN = smtp.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=Business Center/OU=Hosted by Comcast Corporation/OU=EliteSSL/CN=smtp.comcast.net
i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO High-Assurance Secure Server CA
1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO High-Assurance Secure Server CA
i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
2 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=Business Center/OU=Hosted by Comcast Corporation/OU=EliteSSL/CN=smtp.comcast.net
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO High-Assurance Secure Server CA
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: DH, 1024 bits
---
SSL handshake has read 5023 bytes and written 567 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1.2
Cipher : DHE-RSA-AES256-SHA
Session-ID: F9C5D8B413065E5D1296D0A956E8BC6703C935B8D8F20CE60A0211E577907EA5
Session-ID-ctx:
Master-Key: 70AE4C646038BE5C3FD54FA1B748FD2B1836DC4D7ABD718A0CEF604219FDB588ACF740CBBC4934822D1DB92C9309D7C9
Key-Arg : None
PSK identity: None
PSK identity hint: None
SRP username: None
TLS session ticket lifetime hint: 300 (seconds)
TLS session ticket:
0000 - 81 57 af d8 2c 70 20 f6-b8 1c a2 f5 3b bf 9d 35 .W..,p .....;..5
0010 - c5 98 1d 73 75 78 9f 62-3d 0d 94 15 9e 1c 15 aa ...sux.b=.......
0020 - f9 71 75 51 79 64 c6 81-70 fa d6 85 88 a8 81 04 .quQyd..p.......
0030 - 28 9d b3 53 2e 01 38 dd-83 64 6c bf 64 4e 41 66 (..S..8..dl.dNAf
0040 - a1 21 b3 21 83 96 90 1f-63 e6 dd 57 25 0d 36 3f .!.!....c..W%.6?
0050 - cb bd 13 b4 e4 41 88 92-20 43 ee 6d a3 29 a1 63 .....A.. C.m.).c
0060 - 94 19 99 46 f2 4a 9d 94-23 cc d0 f2 47 12 f4 62 ...F.J..#...G..b
0070 - ee 1f 5f 88 ae ab a9 c4-ea b9 fe ef 7d 5f e4 00 .._.........}_..
0080 - 0f 00 29 fb c6 7e a0 b0-07 9e a8 ea cf d7 49 42 ..)..~........IB
0090 - 69 7c d8 0e c9 c8 59 ad-3f 14 6b 75 8e bb 5b b5 i|....Y.?.ku..[.

Start Time: 1435714393
Timeout : 300 (sec)
Verify return code: 0 (ok)
---
250 OK

So now we know why.  Doubt if we will see a solution any time soon.