From the USPS Mail Service Updates page:
The Postal Service has suspended mails service to Syria due to transportation issues.
Until further notice, all Post Office and other acceptance locations will not accept mail destined to Syria.
The suspension affects Express Mail International, Priority Mail International, First-Class Mail International, First-Class Package International Service, International Priority Airmail (IPA) International Surface Air Lift (ISAL), and M-Bag shipments.
Mail addressed to Syria will be returned to sender endorsed “Mail Service Temporarily Suspended — Return to Sender.”
Upon request, postage and fees will be refunded, or mailers may opt to hold and later re-mail the package with existing postage when service to Syria resumes. When re-mailing, customers must strike the endorsement “Mail Service Temporarily Suspended — Return to Sender.”
The Postal Service appreciates its customers’ business and regrets any inconvenience posed by this temporary service suspension.
Customers are advised to periodically check Mail Service Updates for updated information.
Read more: Mail Service Updates.