Why are non-subscribers receiving marketing emails and submitting unsubscribe requests?

Email marketer from ActiveCampaign explains that signup forms on partner sites sometimes don't make it clear you are signing up for multiple lists. People may not realise they are subscribing to *your* marketing emails.

Marketer from Email Geeks notes that not all Mailbox Providers (MBPs) use/honor DMARC in the same way and asks if there is a correlation with who is using the form.

Email marketer from StackExchange explains that poor list hygiene results in inaccurate data, and can cause emails to be sent to addresses that didn't opt-in or are no longer valid subscribers.

Email marketer from Neil Patel Blog explains that a potential reason non-subscribers receive emails is due to data breaches or purchased lists, where email addresses are illegally obtained and used for marketing purposes.

Email marketer from Mailchimp explains that 'form stuffing' or scraping can add unintended emails to a list. Bots can fill out forms, adding emails without consent.

Email marketer from Reddit explains that typos in the sign-up form can result in emails sent to the wrong address if a user makes a mistake, but it's close enough to a real address to be delivered.

Email marketer from Sendinblue explains that email forwarding (user has a forward rule) and list bombs (someone signs you up for many lists at once) can make it seem like you are sending to non-subscribers. Additionally, using multiple lists might overlap.

Email marketer from Email on Acid explains that outdated email lists are problematic. People change email addresses, forget about opting in, or never opted in. Sending to outdated lists will lead to complaints.

Expert from Email Geeks suggests that if the unsubscribe requests coincide with email sends, the message complained about is likely one that was sent by Allie.

Expert from Spamresource explains how confirmation messages help users confirm their intention of getting mail which helps with data quality.

Expert from Word to the Wise explains that incorrectly configuring address harvesting can result in emails being sent to role accounts (e.g. postmaster@, abuse@) or addresses that are similar to, but not actually, the intended subscriber's address.

Expert from Email Geeks suggests two likely causes: either the emails are being forwarded, or the list being checked for subscribers is not the list being mailed to.

Expert from Email Geeks suggests that if full headers are unavailable, tracking links, unsubscribe links, or personalization can be used to work out the email address the content was sent to.

Expert from Email Geeks suggests obtaining a copy of the email that the recipients are complaining about, ideally with full headers, to investigate the issue.

Documentation from RFC Editor details issues of unsolicited email, explaining that improperly configured systems or compromised accounts can lead to sending email to unintended recipients.

Documentation from Microsoft explains that if email accounts are compromised, attackers can use them to send out spam, leading to emails being sent to people who never subscribed.

Documentation from Google details its spam policies, stating that sending unsolicited emails violates its terms, leading to penalties that can cause unintended recipients to receive marketing emails due to deliverability issues.