STARTTLS not exist when telnet smtp.office365.com

Question

STARTTLS not exist when telnet smtp.office365.com

Ardan Zaki 236

Hi everyone,

I have 4 Exchange server with hybrid environment deployed. I have 2 server that doesn't get starttls when I tried to telnet smtp.office365.com 25, while 2 other servers are have that starttls.

anybody has the same issue or solution for that?

This one from the server that doesn't get the starttls

no starttls

This is from the server that get starttls.

starttls

The problem is the send connector to office 365 need starttls from the server that in the scoping of that send connector.

I have already enable the tls 1.2 and disable 1.0 and 1.1

2 answers

Your answer

Answer 1

Kael Yao-MSFT 37,721 Microsoft External Staff

Hi @Ardan Zaki

Please check this Microsoft documentation and see if it may apply to this issue:

Can't receive mail in a hybrid environment after you install a new certificate on the server

Refer to this part:

If this is the cause of this issue, please follow the resolution part to enable SMTP service on the new certificate.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Ardan Zaki 236 Reputation points

2023-06-20T02:26:04.55+00:00

Hi, thank your for replying my question,

All Server Applying the same certificate (2 DC and 2 DRC)

But it works only for 2 DRC server, while 2 DC server is don't get starttls on port 25.

FYI, the starttls is available on 587 only (DC server) but doesn't work with port 25.
Kael Yao-MSFT 37,721 Reputation points Microsoft External Staff

2023-06-21T02:45:43.93+00:00

Hi,

Thanks for the information.

While since this issue only occurs to servers in the same datacenter, another possible cause may be firewalls in the network blocking SMTP packets.

Please have a check if it may be the cause in this case.

To narrow down this issue you can also trying telnet other external email service on port 25 and see if it is also missing STARTTLS. (as in the screenshot of my former reply)
Kael Yao-MSFT 37,721 Reputation points Microsoft External Staff

2023-06-26T08:03:28.8433333+00:00

Hi @Ardan Zaki

I am writing here to confirm with you how thing going now?

Please let us know if you would like further assistance.
Ardan Zaki 236 Reputation points

2023-07-05T09:08:50.03+00:00

Hi, I will check that and will confirm you soon.

Thank you
Kael Yao-MSFT 37,721 Reputation points Microsoft External Staff

2023-07-06T01:12:09.42+00:00

Thanks for your update.

Please feel free to post back.

Answer 2

Amit Singh 5,231

Check this similar thread for help - https://answers.microsoft.com/en-us/msoffice/forum/all/smtp-server-doesnt-support-starttls/3bf73694-75f4-49e6-9bb8-baa8132677e5

Ardan Zaki 236 Reputation points

2023-06-20T07:40:29.5166667+00:00

Hi, I appreciate it, but it seems not related to my issue

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STARTTLS not exist when telnet smtp.office365.com

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