friendly reminder

• heads upAdvance warning notice
• noticeBecome aware of
• memoBrief internal message
• emailDigital written communication
• follow-upSubsequent action on something

gentle nudge · courteous · tactful · professional · deadline · overdue · outstanding · calendar · notification · ping · check-in · attention · please note · don't forget · subtle hint · cordial · inbox · pending

• adviceadvice, advise, tips, counsel, guidance
• suggestion phraseshow about, why not, consider this, what if, perhaps