Archive mails in Apple Mail plus Office 365 setup I am using my company mailbox since 2011. It has 18gb and >120.000 emails. I feel it very slow since several weeks and would like to archive a bunch of mails. Whats the right approach for archiving in an Apple Mail + Exchange (Office365) environment?


I would prefer to keep the mails online, not on any of my machines
I would like to automate it
I expect to significantly increase search speed on normal searches...
... but to be able to search through archived emails as well


Looking forward to your input. Thanks!
Sorry for not explaining much in detail considering that the questions are self-explanatory and the links I added had the rest of the solutions.
As been already explained, the mails have heaped to a huge bulk, probably, thus filling up the inbox and slowing down your account. Archiving them all is definitely a good option. 
This article will explain you how to archive your mails for Apple mail:
https://support.apple.com/en-in/guide/mail/use-the-archive-mailbox-mail35918/mac
Avoiding machine storage leaves you to options like Dropbox and Google drive.
I recommend Google drive for its unlimited storage capacity. You need to install google drive on your system. 
This thread speaks how to save your emails to google drive:
https://productforums.google.com/forum/#!topic/gmail/gCc99XgqMzw;context-place=forum/gmail/categories
This manual process carries the chances of losing out some of your mail contents. You can use Mail Backup X for the job. It sets the right sync between your mails and the storage location.