Since February, I have received over fifty "Delayed Mail (still being retried)" notices from [email protected], which say that an email I sent 12 hours prior hasn't been delivered yet, and will be retried up to 2 days. I have received five "Undelivered Mail Returned to Sender" notices, meaning five out of the fifty could not be delivered even after retrying for 48 hours.
I think this is some kind of tarpitting or graylisting, but since it has affected multiple aliases and a wide variety of recipient domains, it seems to be something Proton is doing to my free-tier account. I am reporting it here as an SL issue because, as a test, I BCC'ed one email to a 10minutemail inbox, without using an SL alias to mask my Proton email. That one email was delivered immediately. My normal practice is to use an alias all the time, and to never expose my Proton email. So I think most or all my emails are being delayed, but I only get notified when the delay exceeds 12 hours.
All my outgoing email are brief, strait-to-the-point, KISS-style, plaintext messages. None of it is marketing or even transactional.
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message/delivery-status
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Reporting-MTA: dns; mail-4321.protonmail.ch
X-Postfix-Queue-ID: 4fmYq26bV7zMr70l
X-Postfix-Sender: rfc822; [REDACTED]@proton.me
Arrival-Date: Thu, 2 Apr 2026 07:47:26 +0000 (UTC)
Final-Recipient: rfc822; [REDACTED]@simplelogin.co
Original-Recipient: rfc822;[REDACTED]@simplelogin.co
Action: delayed
Status: 4.2.2
Remote-MTA: dns; mx1.simplelogin.co
Diagnostic-Code: smtp; 452 4.2.2
<[REDACTED]@simplelogin.co>: Recipient address
rejected: Too many emails delivered to the same user
Will-Retry-Until: Sat, 4 Apr 2026 07:47:26 +0000 (UTC)
Final-Recipient: rfc822; [REDACTED]@simplelogin.co
Original-Recipient: rfc822;[REDACTED]@simplelogin.co
Action: delayed
Status: 4.2.2
Remote-MTA: dns; mx1.simplelogin.co
Diagnostic-Code: smtp; 452 4.2.2
<[REDACTED]@simplelogin.co>: Recipient
address rejected: Too many emails delivered to the same user
Will-Retry-Until: Sat, 4 Apr 2026 07:47:26 +0000 (UTC)
Since February, I have received over fifty "Delayed Mail (still being retried)" notices from [email protected], which say that an email I sent 12 hours prior hasn't been delivered yet, and will be retried up to 2 days. I have received five "Undelivered Mail Returned to Sender" notices, meaning five out of the fifty could not be delivered even after retrying for 48 hours.
I think this is some kind of tarpitting or graylisting, but since it has affected multiple aliases and a wide variety of recipient domains, it seems to be something Proton is doing to my free-tier account. I am reporting it here as an SL issue because, as a test, I BCC'ed one email to a 10minutemail inbox, without using an SL alias to mask my Proton email. That one email was delivered immediately. My normal practice is to use an alias all the time, and to never expose my Proton email. So I think most or all my emails are being delayed, but I only get notified when the delay exceeds 12 hours.
All my outgoing email are brief, strait-to-the-point, KISS-style, plaintext messages. None of it is marketing or even transactional.