There should be the possibility to add any sender or recipient to your contacts.
When it’s the sender, we could also take into account the signature of the mail and extract any info like phone numbers or addresses there.
As @jancborchardt said it in the old archived issue: https://github.com/owncloudarchive/mail/issues/363
As @jancborchardt said it in the old archived issue: https://github.com/owncloudarchive/mail/issues/363