Bounce Handling
Bounces occur if an email server answers with a status code in the 4xy or 5xy range. 4xy status codes lead to "soft bounces" whereas 5xy status codes lead to "hard bounces".
Soft bounces happen frequently and are expected, especially for greylisting. The p25.dev email servers will try to deliver the same email again at a later moment. If the same email keeps on soft-bouncing for 7 days, we discard the email and send the original sender a nondelivery report (NDR).
Hard bounces happen if the receiving email server immediately decides that it will not accept the email, not even if delivered at a later time. In this case, p25.dev email servers immediately give up, discard the email and send the original sender a nondelivery report, with one exception in the case of Spam reporting (see below).
Rewriting MAIL FROM
If you send an email from your email server via p25.dev, your email server will tell us something like
MAIL FROM: <hello@example.com>.
If we needed to send a bounce to you, we would use this address to notify you about the bounce.
If we take your email and forward to another email server, we use our own MAIL FROM: <{dynamic}@bounce.p25.dev> address.
The {dynamic} part of the address tells us internally that your original address was hello@example.com.
The protocol is
called Sender Rewriting Scheme.
For us, it is important to see the bounce first. This way we can calculate your bounce rate and decide if we need
to temporarily store the receiving address in the suppression cache (see below for an explanation).
We will forward the bounce message to you then.
Bounce rate
Note: You see your current bounce rate on your Dashboard.
If the emails you send via p25.dev are exceeding a certain amount of bounces, we need to temporarily suspend sending for your account. A high bounce rate is generally a sign of a wrong configuration, guessing addresses or using an outdated address list.
It is your responsibility to keep your bounce rate below the threshold of 5%.
The first 10 bounces are granted without counting towards your bounce rate. Beyond this number, we look up how many successful deliveries you had vs. how many emails bounced. If for all sent emails in the past 24h the bounce rate exceeds 5%, we temporarily suspend your account.
Don't worry, things happen, and we unblock your account automatically after some cool-down period. If you have high bounce rates frequently, we reserve the right to terminate your subscription.
The bounce rate is only calculated for emails you send, not for the emails you receive.
Suppression cache
Note: You see all suppressed addresses on your Dashboard.
In some situations, it does not make sense to send an email again to the same address, for example if the address does not exist on the receiving email server. In this case, we temporarily store this address in a suppression cache. Our mail servers will immediately reject emails for the same address, because we already know that this email cannot be delivered.
This is to protect our reputation. Repeatedly trying to deliver emails to non-existing addresses is broadly seen as being "spammy".
Suppressed addresses will generally expire, and you can try again to email this address.
The suppression cache works for inbound and outbound emails. If your email server hard-bounces an email address, we will not accept emails for this address for a while to make sure we don't need to send unnecessary NDRs.
Spam bounces
Note: This is NOT related to emails being classified as "Spam" in your inbox.
If your email server hard-bounces an email because it suspects Spam, we will not send an NDR to the original sender. Spammers oftentimes fake the sender address. We prevent sending NDRs to addresses that are likely not involved in sending those emails in the first place.
Testing bounces
If you want to test certain behavior of your email server, you can provoke different bounces at bounce.p25.dev.
The addresses are:
421@bounce.p25.dev451@bounce.p25.dev500@bounce.p25.dev550@bounce.p25.dev