Allow SSL /TLS in email outbound SMTP settings

[necrevistonnezr]

Currently, I think I can only use STARTTLS as a protocol when setting up an external SMTP relay.

My email provider mailbox.org, however, uses SSL / TLS on Port 465: https://kb.mailbox.org/display/MBOKB/E-Mail-Einrichtung+-+allgemein (https://translate.google.com/translate?hl=de&sl=de&tl=en&u=https%3A%2F%2Fkb.mailbox.org%2Fdisplay%2FMBOKB%2FE-Mail-Einrichtung%2B-%2Ballgemein)

Setting up smtp.mailbox.org as an SMTP relay therefore fails.

[girish]

@necrevistonnezr Try smtp.mailbox.org at port 587 for STARTTLS. Generally, both are equally secure.

[necrevistonnezr]

@girish Thanks, that worked - I didn't know mailbox.org worked at Port 587 as well.

Now I have a weird error, though. I had to configure email addresses from my cloudron domain as email aliases (inter alia no-reply@cloudrondomain.tld) in mailbox.org so that I can send mails via such cloudron domain addresses.
However, when I send a test email from cloudron to my existing mailbox.org address, I get the following error:

This is the mail system at host mout-f-109.mailbox.org.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<user@cloudrondomain.tld>: host my.cloudrondomain.tld[###.###.###.###] said: 550 Mail from domain
    'cloudrondomain.tld' is not allowed from your host (in reply to MAIL FROM command)
Reporting-MTA: dns; mout-f-109.mailbox.org
X-Postfix-Queue-ID: 48f2qt0MP8zKnH4
X-Postfix-Sender: rfc822; no-reply@cloudrondomain.tld
Arrival-Date: Fri, 13 Mar 2020 12:03:38 +0100 (CET)

Final-Recipient: rfc822; user@cloudrondomain.tld
Original-Recipient: rfc822;user@cloudrondomain.tld
Action: failed
Status: 5.0.0
Remote-MTA: dns; my.cloudrondomain.tld
Diagnostic-Code: smtp; 550 Mail from domain 'cloudrondomain.tld' is not allowed from your host

(domains and IP changed for privacy)

[girish]

Are you hosting a custom domain on mailbox.org or do you have a @mailbox.org address? If it's the latter, mailbox is then not really an email relay. Generally, email relays are able to forward all addresses of a domain i.e anything@cloudrondomain.tld.