This doesn’t happen all the time but occasionally the reply all button will be missing and I am unable to reply to everyone on an email. Does anyone know why this might be happening?

Could be basically anything: Browser add on like ublock, reverse proxy rule, maybe only one recipient on the mail to answer…
We can only guess what client software you are using…

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    Guess @realizelol talking about SoGo, On our server users raised this ticket multiple times, answer was always: In mail was only one recipient even if previous mails in thread has multiple because someone in chain did not click reply all,..

    esackbauer Using SoGo

    So if we send an email to multiple people, then go to my sent items I am unable to reply all to everyone the email was sent to.

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