Emails not received

We run on-premise gitlab enterprise 13.12.00-ee and it used to deliver emails but stopped at some point, with no changes on the gitlab host whatsoever.

It is configured to use an IP address of the SMTP server, port 25, plain password auth, and optional STARTTLS.

gitlab_rails['smtp_enable'] = true
gitlab_rails['smtp_address'] = ""
gitlab_rails['smtp_port'] = 25
gitlab_rails['smtp_user_name'] = "me@..."
gitlab_rails['smtp_password'] = "..."
gitlab_rails['smtp_domain'] = "..."
gitlab_rails['smtp_authentication'] = "login"
gitlab_rails['smtp_enable_starttls_auto'] = true
gitlab_rails['smtp_openssl_verify_mode'] = 'peer'
gitlab_rails['smtp_email_from'] = "me@..."
gitlab_rails['smtp_email_reply_to'] = "noreply@..."

I verified that the address and credentials are correct and can be used in an email client.

I ran telnet and openssl and successfully sent an email from the gitlab box, using the address, port#, and credentials from gitlab.rb.

In production.log, I can see

[ActiveJob] Enqueued ActionMailer::MailDeliveryJob (Job ID: ...) to Sidekiq(mailers) with arguments: "DeviseMailer", "reset_password_instructions", "deliver_now", {:args=>[#<GlobalID:0x... @uri=#<URI::GID gid://gitlab/User/6>>, "...", {}]}

entry but nothing else that is immediately obviously related to sending emails.

In the SMTP server logs I can see the welcome header, received EHLO, received STARTTLS. This is the end of it, so no further commands seem to be sent by gitlab.

Out of ideas at this point.