Why am I seeing temporary server errors on 0365 for specific subnets?

Email marketer from Mailjet shares that new IPs or IPs with low sending volume can encounter temporary server errors due to trust issues. Warming up IPs gradually by increasing sending volume over time can improve deliverability.

Email marketer from StackOverflow explains that Office 365 imposes rate limits on incoming emails from specific sources (subnets). If you exceed these limits, you might encounter temporary server errors until the rate limit resets.

Email marketer from Email Geeks explains that subnet blocks may not always show up in query tools and that the underlying issue might be a security block and also explains that they are supposed to auto expire.

Email marketer from StackExchange suggests that temporary server errors specifically tied to certain subnets could indicate a problem with the IP reputation of those subnets. Email service providers might be throttling or temporarily blocking emails from IPs with a history of spam or low engagement.

Email marketer from EmailToolTester explains that there are many free tools available online to check your mail server's or IP address against many of the most well-known email blacklists to see if this is causing any of your issues.

Email marketer from Quora shares that if the subnet in question uses shared IP addresses for sending email, the actions of other senders on the same IP could negatively impact deliverability and lead to temporary errors with certain recipients, including Office 365.

Email marketer from Email Deliverability Forum suggests that incorrect or incomplete SPF or DKIM records, especially those related to the sending subnets, can cause receiving mail servers like Office 365 to temporarily reject emails. Ensuring these records are properly configured for all sending IPs is crucial.

Marketer from Email Geeks mentions that Microsoft uses Talos as a data feed, suggesting that the subnet might have a historical problem listed there.

Email marketer from Reddit explains that Microsoft Office 365 might be implementing subnet-specific throttling due to perceived abuse or policy violations originating from that subnet. This can manifest as temporary server errors during peak sending times.

Email marketer from Superuser suspects that the sending IP addresses within the subnet could be listed on one or more email blocklists (blacklists). This could cause temporary rejections from Office 365.

Email marketer from Email Deliverability Blog explains that if the reverse DNS (rDNS) records for your sending IPs (within the specific subnets) are missing, incorrect, or do not match the forward DNS records, some mail servers, including Office 365, might temporarily reject connections.

Expert from Spam Resource explains that greylisting, a spam-fighting technique, could be the cause. Greylisting works by temporarily rejecting emails from unknown senders. A legitimate server will try again after a delay and be accepted, while a spammer may not.

Expert from Word to the Wise shares that subnet reputation can be problematic, especially with cloud providers. If other users on your subnet are sending spam, it can negatively impact the reputation of the entire subnet. This means temporary errors from services like 0365.

Expert from Email Geeks suggests that the error might actually be temporary and advises to use a subnet that works as a workaround while looking for the root cause.

Expert from Email Geeks suggests examining the MTA configuration, sniffing traffic for unusual TLS negotiation, checking EHLO values, and reviewing rDNS records if Microsoft says there's no IP block.

Documentation from Microsoft Learn explains that temporary errors in Exchange Online, often indicated by 4xx or 5xx SMTP error codes, can arise from transient network issues, server overload, or maintenance activities. These errors suggest the sender should retry delivery after a certain period.

Documentation from SendGrid explains that implementing proper retry logic in your email sending infrastructure is essential to handle temporary server errors. This involves automatically retrying failed deliveries after a delay, with an exponential backoff strategy to avoid overwhelming the receiving server.

Documentation from MXToolbox explains that temporary DNS resolution problems might result in temporary server errors. If the sending server cannot reliably resolve the MX records for the recipient domain (0365 in this case), it might temporarily defer the email.

Documentation from RFC standard states that SMTP servers use 4xx error codes to indicate temporary failures. These failures suggest that the client should retry the request later, as the server is currently unable to process it but may be able to do so in the future.

Documentation from Microsoft Learn states that transient issues with SMTP connections, such as those caused by network congestion or brief outages, can trigger temporary server errors. These are often automatically retried by the sending server.