There are many ways to finish a letter/e-mail.

Le meas = with respect (sounds better in Irish)
Is mise le meas = I am yours respectfully - not sure how well the English versions sound....
Le gach dea-ghuí = with all good wishes
Beannachtaí = blessings
Míle buíochas = many thanks (literally - a thousand thanks)
Beir bua = good luck

Does anyone else have any others to add?

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ádh mór = good luck
go n-éiridh leat = may you succeed

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

This is taken from Collins Pocket Dictionary -
FORMAL

Mise, Le meas,

LESS FORMAL

Beir bua agus beannacht,
Le dea-mhéin,
Rath Dé ort,

INFORMAL

Beannacht,
Slán go fóill,
Ádh Mór,
Grá mór,
Do chara,
Le grá ó

And go n-éirí an t-ádh leat

Go raibh maith agaibh go léir. Tá rogha mór againn anois. An cheann is fearr liom ná 'Beir bua agus beannacht'. Ní úsáidim é sin, ach caithfidh mé feasta. Is maith liom é.

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