I know that iOS 18 is now a couple of versions old at the time of this posting however the issue I was experiencing with it is still present in the current version (presently 26.4).
Summary of problem: iOS (and iPad OS) devices running version 18.x and up are unable to check email when configured to use IMAP services. The problem appears to have been limited to a small number of systems. The problem was not present in iOS 17.x and earlier nor did it affect secure POP3 or outgoing SMTP. Furthermore the issue did not affect macOS versions (nor any other IMAP client I used), it appeared to be limited only to the mail application on iOS / iPad OS.
My situation: I run a small mail server for family and friends. As they upgraded their devices to iOS 18 they experienced this issue. As I am the administrator I was able to perform some detailed investigation (some of which is documented elsewhere). In the end I was unable to determine why the problem persisted. As such I remained on iOS 17 while others switched to using POP3.
Resolution: I obtained an SSL certificate from a certificate authority (as opposed to using the self-signed certificates I had been using until now) and the issue appears to have been resolved. Aside from using this new certificate nothing else changed with my configuration. Something about Apples iOS mail application does not agree with self-signed certificates when using the IMAP protocol.
While this affected a small number of users and iOS 18.x has been superseded I decided to post here because 1) some devices cannot be upgraded past version 18 and 2) it affects all subsequent versions. This may not solve every instance of this issue but I wanted to provide the solution that fixed my issue with the hope it will help others who might stumble across this issue.
Summary of problem: iOS (and iPad OS) devices running version 18.x and up are unable to check email when configured to use IMAP services. The problem appears to have been limited to a small number of systems. The problem was not present in iOS 17.x and earlier nor did it affect secure POP3 or outgoing SMTP. Furthermore the issue did not affect macOS versions (nor any other IMAP client I used), it appeared to be limited only to the mail application on iOS / iPad OS.
My situation: I run a small mail server for family and friends. As they upgraded their devices to iOS 18 they experienced this issue. As I am the administrator I was able to perform some detailed investigation (some of which is documented elsewhere). In the end I was unable to determine why the problem persisted. As such I remained on iOS 17 while others switched to using POP3.
Resolution: I obtained an SSL certificate from a certificate authority (as opposed to using the self-signed certificates I had been using until now) and the issue appears to have been resolved. Aside from using this new certificate nothing else changed with my configuration. Something about Apples iOS mail application does not agree with self-signed certificates when using the IMAP protocol.
While this affected a small number of users and iOS 18.x has been superseded I decided to post here because 1) some devices cannot be upgraded past version 18 and 2) it affects all subsequent versions. This may not solve every instance of this issue but I wanted to provide the solution that fixed my issue with the hope it will help others who might stumble across this issue.