Re: [exim] SMTP vs Submission (port 25 vs 587)

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Author: Marc Sherman
To: exim-users
Subject: Re: [exim] SMTP vs Submission (port 25 vs 587)
Wakko Warner wrote:
> I've been searching for information about why it is a good idea to use 587
> instead of 25 but have not found anything convincing. I have hit a brick
> wall where I work since they think that 587 is non-standard (Even after
> showing them RFC 4409) and thus is not wise. Can anyone provide me with any
> links or anything that would argue this case? Unfortunately, email archives
> are not valid for this.

The two big reasons are:

1) Lots of ISPs block outgoing 25, so if you want to accept incoming
authenticated submission from remote users on arbitrary connections,
port 25 might not be adequate.

2) If MSA and MTA are on different ports, it's easier to set up
different configs for the two use cases. This reason only makes sense if
you want to restrict all submission to port 587, and only allow MTA
traffic on port 25.

- Marc