Currently the effective attachment size for emails sent from Outlook via HTMO is under 20MB for sending (known limitation https://help.hcltechsw.com/traveler/11.0.0/htmo/3.0/use_HTMO_mail_issues.html) and almost 100MB for receiving attachments (the value of "Maximum email Attachment Size Allowed - Administrator" cannot be set to value bigger than 99999).
This doesn't match what user's would expect. Especially they sometimes don't get an error when sending attachments with a size of 20MB, they just stick in the Outlook outbox.
Please raise these values or allow administrators to set bigger ones. It was always a strength of Domino to cope with big email attachments.