Mail server / service reputation:
Whatever state of art techniques are applicable, an IPv4 address needs to build up a constant reputation because of spam filtering world wide. Note: Mail users at limited budget providers have no folder to search in spam, so mail delivery requires stabilization. A mailserver like Google’s IPv6 mail server requires both SPF and DKIM authorization. In this case a mail server using SRS, without DKIM authorization, needs to send from an IPv4 address.
Mailserver using SRS (from wikipedia.org):
For a mail transfer agent (MTA), the Sender Rewriting Scheme (SRS) is a scheme for rewriting the envelope sender address of an email message, in view of remailing it. In this context, remailing is a kind of email forwarding. SRS was devised in order to forward email without breaking the Sender Policy Framework (SPF), in 2003.
Spam scores differ, e.g. for mail to my internet provider xs4all.nl:
server / VPS itself: -0.6
spamport.com mail service (SRS; dmarc=false/pass; DKIM yes/no): -0.6
mailchannels.net mail service (SRS; dmarc=false/pass; DKIM yes/no; no DNSSEC): -0.1
transip.nl VPS mail service: -0.1
sparkpost.com email delivery service: -0.1
amazon.com Simple Email Service using SMTP: -0.6
Note: the HTTP process at Amazon results in spam score less good.
Forward of forward to a non-local mailbox may block in Roundcube:
(and forwarding as setup to a non-local mailbox then ends in the mail queue)
server / VPS itself: pass
spamport.com mail service (with SRS): pass
mailchannels.net mail service (with SRS): pass
transip.nl VPS mail service: fail
sparkpost.com email delivery service: fail
amazon.com Simple Email Service: fail
Some ways to setup mail:
server / VPS itself: MTA(Postfix/Exim – SRS works when needed) / SMTP
transip.nl VPS mailservice (no SRS): MTA(Postfix/Exim)
cyberfusion.nl mailcluster (no SRS anymore): MTA(Postfix/Exim)
sparkpost.com email delivery service: MTA(Postfix/Exim) / SMTP / HTTP with library
amazon.com Simple Email Service (with SRS): MTA(Postfix/Exim) / SMTP / HTTP with library
Location mail server:
amazon.com Simple Email Service: one of the mail servers is in Ireland
mailchannels.net mail service: mail servers in U.S. and one in Switzerland
sparkpost.com email delivery service: mail server move from U.S. to Ireland
(as should be by the end of March 2018)
Hint for MTA configuration:
Configure Exim for relay with three extra files. Standard configuration can be customized this way.