1. In a sporadic manner
   - sporadically 
2. At regular time intervals

See also: periodical

• mail call
• mail car
• mail carrier
• mail clerk
• mail fraud
• mail merge
• mail order
• mail out
• mail pouch
• mail service
• mail slot
• mail train
• mail-cheeked
• mail-clad
• mail-order buying
• mailbag
• mailboat
• mailbox
• maildrop
• mailed
• mailer
• mailing
• mailing address
• mailing list
• mailing-card