The reporting web service and/or PowerShell are your options here. Nothing regarding mail flow is exposed within the Graph API, either under the reporting endpoints or otherwise. Get-MessageTrace should get you started and also these:
Get-Command Get-Mail*Report
Get-MailboxActivityReport
Get-MailboxUsageDetailReport
Get-MailboxUsageReport
Get-MailDetailATPReport
Get-MailDetailDlpPolicyReport
Get-MailDetailEncryptionReport
Get-MailDetailEvaluationModeReport
Get-MailDetailTransportRuleReport
Get-MailFilterListReport
Get-MailFlowStatusReport
Get-MailTrafficATPReport
Get-MailTrafficEncryptionReport
Get-MailTrafficPolicyReport
Get-MailTrafficSummaryReport