How can I find out whether my server supports this?
You have to connect to your mailserver and ask it, I suppose. There are two ways to quickly achieve that, assuming you are familiar with using the CLI.
EDIT: You'll need to look for the capability "CONDSTORE". You can do this by logging in as described below, or by looking into the main log in FairEmail as described by Marcel. Using the app will be easier.
1. Telnet
Simply open a telnet client and connect to the IMAP server (no SSL/TLS), so possibly on port 143. You will receive a list of pre-login capabilities. Then enter the following:
Code:
tag login <username> <password>
Now you should get a list of all capabilities and can send requests to your mailserver afterwards. Have fun seeing what FairEmail has been doing all the time in the background to make us all happy
Fun aside, you might want to test using the example as descriped in
Server Implementation Consideration (RFC 4551). So try out this command:
Code:
tag SELECT INBOX
tag STORE 2:4 (UNCHANGEDSINCE 2021040800000) +FLAGS.SILENT ($Processed)
If the server response includes "MODSEQ", your server does support MODSEQ. Else, it will return a "NOMODSEQ" at some part of it's response as described in
NOMODSEQ Response Code (RFC 4551). While you selected your INBOX, the server might have even responded with HIGHESTMODSEQ. There you have it.
2. OpenSSL
Connect to the mailserver via SSL/TLS, possibly on port 993, using following command:
Code:
openssl s_client -connect <imap-hostname>:<port> -quiet
Now you may log in as described above. To log out, you can use the following command with both methods:
Anyways. Have fun, but don't play around too much. You won't want to enter the wrong commands and delete some mails haha
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P.S.: I am not really that well versed in IMAP myself, so I just read some RFCs and wrote down some things the way it should work (and it did in my case). If there's anything wrong with this approach, feel free to correct me