We have recently applied a number of changes surrounding email deliveries.
Previously, we routed all outbound mail sent via the SMTP protocol through MailChannels, ensuring that all SMTP messages were delivered to the end users / recipients.
However, one thing we noticed over the span of several months was that much of the 'SPAM' relayed via the PHPMail() protocol caused a number of our IP addresses and subnets to be 'blacklisted' - to combat this, we have reinvested significantly to now route all outbound mail through MailChannels, so that we can benefit from the outbound SPAM filtering that bundles with the solution.
In turn, this has allowed us to ensure that the IP addresses in our subnets are far less likely to be blacklisted.
We still highly recommend using the SMTP protocol for relaying all messages however, as its still possible for mail sent via PHPMail to be treated as SPAM - not due to the impact of blacklists, but instead due to the way the 'envelope senders' are relayed.
Additionally, we have increased our outbound sending limits per-account to 150 from the previous 100